Here is another version that controls the appearance of the rule with various pgf keys and draws the thing in one stretch.
\documentclass{article}
\usepackage{lipsum}
\usepackage{tikz}
\usetikzlibrary{fadings}
\tikzfading[name=fade out,
left color=transparent!100,right color=transparent!100,
middle color=transparent!0]
\tikzset{faded rule style/.style={},faded rule/.cd,color/.initial=black,width/.initial=\textwidth,
height/.initial=1ex,style/.code=\tikzset{faded rule style/.style={#1}}}
\newcommand{\FadeRule}[1][]{\noindent\tikz{\def\pv##1{\pgfkeysvalueof{/tikz/faded rule/##1}}
\tikzset{faded rule/.cd,#1}
\path(-\textwidth/2,0) -- (\textwidth/2,0);
\fill[faded rule style,color=\pv{color},path fading=fade out] (-\pv{width}/2,-\pv{height}/2)
rectangle (\pv{width}/2,\pv{height}/2);}}
\begin{document}
\lipsum[1]
\FadeRule
\lipsum[2]
\FadeRule[color=blue,height=1em]
\lipsum[3]
\FadeRule[color=red,width=0.7\textwidth]
\lipsum[3]
\FadeRule[style={opacity=0.5}]
\end{document}
