I have created a figure for tikz that looks like this:

But when I add it inside a matrix node it became like this:

Why does this happen?
My code for the first is:
\begin{tikzpicture}
\node[](users){\includegraphics[scale=0.5]{icons/user.png} };
\node[below of=users,yshift=-20](google){\includegraphics[scale=0.3]{icons/googleplus.png} };
\node[below left of=users,yshift=-10](facebook){\includegraphics[scale=0.3]{icons/facebook.png} };
\node[below right of=users,yshift=-10](twitter){\includegraphics[scale=0.5]{icons/tweets.png} };
\draw[->,line width=1] (users)--(facebook);
\draw[->,line width=1] (users)--(google);
\draw[->,line width=1] (users)--(twitter);
\node[below of=google,yshift=8]{Content};
\end{tikzpicture}
My code for the second is:
\begin{tikzpicture}
\tikzstyle{mybox} = [draw=red, fill=blue!20, very thick,rectangle, rounded corners,minimum width=1.5cm,minimum height=3cm]
\tikzstyle{fancytitle} =[fill=red, text=white,rounded corners]
\node [matrix,mybox] (data){
\node[minimum size=2cm]{};
\node[](users){\includegraphics[scale=0.5]{icons/user.png} };
\node[below of=users,yshift=-20](google){\includegraphics[scale=0.3]{icons/googleplus.png} };
\node[below left of=users,yshift=-10](facebook){\includegraphics[scale=0.3]{icons/facebook.png} };
\node[below right of=users,yshift=-10](twitter){\includegraphics[scale=0.5]{icons/tweets.png} };
\draw[->,line width=1] (users)--(facebook);
\draw[->,line width=1] (users)--(google);
\draw[->,line width=1] (users)--(twitter);
\node[below of=google,yshift=8]{Content};
\\
};
\node[fancytitle, right=10pt] at (data.north west) {Data};
\end{tikzpicture}

\nodes inside\nodes? If you are only trying to box the first diagrams, this can be done in different, more appropriate ways. – Gonzalo Medina May 06 '14 at 14:23