Well, there are three issues in your code.
- You did not define the length of
\footskip you need to get a proper placed footer (the value may change depending on the personal informations you are using; check your log file for a warning "Package Fancyhdr Warning: \footskip is too small (30.0pt):. Use a value you need, in my case \setlength{\footskip}{52pt} ...
At last I guess you realy want to use command \socials instead your used \extrainfo. If you insist in using \extrainfo as a workaround simply change the \footsymbol to be a little bit shorter, then it does not write into the page number with your given personal informations:
\renewcommand*{\footsymbol}{{~~{\rmfamily\textbullet}~~}}
But the best solution for your issue is to use command \social. It is able to add your Xing, StackOverflow,Linkedin and Github informations to your personal datas in your footer in the simplest way (standard definitions):
\social[linkedin]{john-doe}
\social[github]{jdoe}
For Stackoverflow and Xing we have to do a little bit more:
\newcommand*{\xingsocialsymbol}{\faXing~}
\social[xing][xing.com/JohnDoe]{JohnDoe}
\newcommand*{\SOfsocialsymbol}{\faStackOverflow~}
\social[SOf][www.StackOverflow.com/profile/john.doe]{john.doe}
Change www.StackOverflow.com/profile/john.doe to your real datas, for example tex.stackexchange.com/users/179843.
- Having a closer look to the used links in your cv, you will see that in the standard definition only
http:// is used instead to today used https://. To solve this issue please have a look to my answer for question https://tex.stackexchange.com/a/464850/16550.
With the following MWE I show you the solutions for issue 1 and two together (see markings in code <========):
\documentclass[11pt,a4paper]{moderncv}
\moderncvtheme[blue]{casual}
\renewcommand{\familydefault}{\sfdefault}
\setlength{\footskip}{52pt}% <==========================================
\renewcommand*{\footsymbol}{{~~{\rmfamily\textbullet}~~}} % <===========
\firstname{John}
\familyname{Doe}
\title{CV}
\address{Somewhere}
\mobile{+1 (800) 555-5555}
\email{my@email.com}
\social[linkedin]{john-doe} % <=========================================
\social[github]{jdoe}
\newcommand*{\xingsocialsymbol}{\faXing~}
\social[xing][xing.com/JohnDoe]{JohnDoe}
\newcommand*{\SOfsocialsymbol}{\faStackOverflow~}
\social[SOf][www.StackOverflow.com/profile/john.doe]{john.doe}
\social[github]{jdoe} % <===============================================
\begin{document}
\maketitle
\newpage
test
\end{document}
and the resulting pdf page:

In your comment you asked "Is there a way to start with the first social on a new line?". Well, you can add the command \flushfoot\\ in the definition of command \makecvfoot where you need it:
\makeatletter
\renewcommand*{\recomputefootlengths}{%
\setlength{\footwidth}{0.9\textwidth}}% <=============================
\renewcommand*{\makecvfoot}{%
\recomputecvfootlengths{}%
\fancypagestyle{plain}{%
\fancyfoot[c]{%
\parbox[b]{\footwidth}{%
\centering%
\color{color2}\addressfont%
\vspace{\baselineskip}% forces a white line to ensure space between main text and footer (as footer height can't be known in advance)
\ifthenelse{\isundefined{\@addressstreet}}{}{\addtofoot[]{\addresssymbol\@addressstreet}%
\ifthenelse{\equal{\@addresscity}{}}{}{\addtofoot[~--~]{\@addresscity}}% if \addresstreet is defined, \addresscity and \addresscountry will always be defined but could be empty
\ifthenelse{\equal{\@addresscountry}{}}{}{\addtofoot[~--~]{\@addresscountry}}%
\flushfoot\@firstfootelementtrue\\}%
\collectionloop{phones}{% the key holds the phone type (=symbol command prefix), the item holds the number
\addtofoot{\csname\collectionloopkey phonesymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@email}}{}{\addtofoot{\emailsymbol\emaillink{\@email}}}%
\ifthenelse{\isundefined{\@homepage}}{}{\addtofoot{\homepagesymbol\httplink{\@homepage}}}%
\flushfoot\\% <===================================
\collectionloop{socials}{% the key holds the social type (=symbol command prefix), the item holds the link
\addtofoot{\csname\collectionloopkey socialsymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@extrainfo}}{}{\addtofoot{\@extrainfo}}%
\ifthenelse{\lengthtest{\footboxwidth=0pt}}{}{\flushfoot}% the lengthtest is required to avoid flushing an empty footer, which could cause a blank line due to the \\ after the address, if no other personal info is used
}}}
\pagestyle{plain}}
\makeatother
With the new complete MWE
\documentclass[11pt,a4paper]{moderncv}
\moderncvtheme[blue]{casual}
\renewcommand{\familydefault}{\sfdefault}
\setlength{\footskip}{52pt}% <==========================================
%\renewcommand*{\footsymbol}{{~~{\rmfamily\textbullet}~~}} % <===========
\firstname{John}
\familyname{Doe}
\title{CV}
\address{Somewhere}
\mobile{+1 (800) 555-5555}
\email{my@email.com}
\social[linkedin]{john-doe} % <=========================================
\social[github]{jdoe}
\newcommand*{\xingsocialsymbol}{\faXing~}
\social[xing][xing.com/JohnDoe]{JohnDoe}
\newcommand*{\SOfsocialsymbol}{\faStackOverflow~}
\social[SOf][www.StackOverflow.com/profile/john.doe]{john.doe}
\social[github]{jdoe} % <===============================================
\makeatletter
\renewcommand*{\makecvfoot}{%
\recomputecvfootlengths{}%
\fancypagestyle{plain}{%
\fancyfoot[c]{%
\parbox[b]{\footwidth}{%
\centering%
\color{color2}\addressfont%
\vspace{\baselineskip}% forces a white line to ensure space between main text and footer (as footer height can't be known in advance)
\ifthenelse{\isundefined{\@addressstreet}}{}{\addtofoot[]{\addresssymbol\@addressstreet}%
\ifthenelse{\equal{\@addresscity}{}}{}{\addtofoot[~--~]{\@addresscity}}% if \addresstreet is defined, \addresscity and \addresscountry will always be defined but could be empty
\ifthenelse{\equal{\@addresscountry}{}}{}{\addtofoot[~--~]{\@addresscountry}}%
\flushfoot\@firstfootelementtrue\\}%
\collectionloop{phones}{% the key holds the phone type (=symbol command prefix), the item holds the number
\addtofoot{\csname\collectionloopkey phonesymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@email}}{}{\addtofoot{\emailsymbol\emaillink{\@email}}}%
\ifthenelse{\isundefined{\@homepage}}{}{\addtofoot{\homepagesymbol\httplink{\@homepage}}}%
\flushfoot\\% <===================================
\collectionloop{socials}{% the key holds the social type (=symbol command prefix), the item holds the link
\addtofoot{\csname\collectionloopkey socialsymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@extrainfo}}{}{\addtofoot{\@extrainfo}}%
\ifthenelse{\lengthtest{\footboxwidth=0pt}}{}{\flushfoot}% the lengthtest is required to avoid flushing an empty footer, which could cause a blank line due to the \\ after the address, if no other personal info is used
}}}
\pagestyle{plain}}
\makeatother
\begin{document}
\maketitle
\newpage
test
\end{document}
you get the resulting footer:

To remove the bullet in front of the linkedin logo you have to add \@firstfootelementtrue after \flushfoot.
With the new MWE
\documentclass[11pt,a4paper]{moderncv}
\moderncvtheme[blue]{casual}
\renewcommand{\familydefault}{\sfdefault}
\setlength{\footskip}{52pt}%
%\renewcommand*{\footsymbol}{{~~{\rmfamily\textbullet}~~}} %
\firstname{John}
\familyname{Doe}
\title{CV}
\address{Somewhere}
\mobile{+1 (800) 555-5555}
\email{my@email.com}
\social[linkedin]{john-doe} %
\social[github]{jdoe}
\newcommand*{\xingsocialsymbol}{\faXing~}
\social[xing][xing.com/JohnDoe]{JohnDoe}
\newcommand*{\SOfsocialsymbol}{\faStackOverflow~}
\social[SOf][www.StackOverflow.com/profile/john.doe]{john.doe}
\social[github]{jdoe} %
\makeatletter
\renewcommand*{\makecvfoot}{%
\recomputecvfootlengths{}%
\fancypagestyle{plain}{%
\fancyfoot[c]{%
\parbox[b]{\footwidth}{%
\centering%
\color{color2}\addressfont%
\vspace{\baselineskip}% forces a white line to ensure space between main text and footer (as footer height can't be known in advance)
\ifthenelse{\isundefined{\@addressstreet}}{}{\addtofoot[]{\addresssymbol\@addressstreet}%
\ifthenelse{\equal{\@addresscity}{}}{}{\addtofoot[~--~]{\@addresscity}}% if \addresstreet is defined, \addresscity and \addresscountry will always be defined but could be empty
\ifthenelse{\equal{\@addresscountry}{}}{}{\addtofoot[~--~]{\@addresscountry}}%
\flushfoot\@firstfootelementtrue\\}%
\collectionloop{phones}{% the key holds the phone type (=symbol command prefix), the item holds the number
\addtofoot{\csname\collectionloopkey phonesymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@email}}{}{\addtofoot{\emailsymbol\emaillink{\@email}}}%
\ifthenelse{\isundefined{\@homepage}}{}{\addtofoot{\homepagesymbol\httplink{\@homepage}}}%
\flushfoot\@firstfootelementtrue\\% <===================================
\collectionloop{socials}{% the key holds the social type (=symbol command prefix), the item holds the link
\addtofoot{\csname\collectionloopkey socialsymbol\endcsname\collectionloopitem}}%
\ifthenelse{\isundefined{\@extrainfo}}{}{\addtofoot{\@extrainfo}}%
\ifthenelse{\lengthtest{\footboxwidth=0pt}}{}{\flushfoot}% the lengthtest is required to avoid flushing an empty footer, which could cause a blank line due to the \\ after the address, if no other personal info is used
}}}
\pagestyle{plain}}
\makeatother
\begin{document}
\maketitle
\newpage
test
\end{document}
you get the wished result:
