Why are all of my footnotes hyperlinked to the titlepage?

In a comment to the question I quoted footmisc's manual:

The hyperref package has ambitions to make hyperlinks from footnote marks to the corresponding footnote body; naturally this causes grief to footmisc, and unfortunately no remedy is currently known. If you use footmisc, suppress hyperref's hyperfootnotes, by loading it as: \usepackage[hyperfootnotes=false,...]{hyperref} Further work on the interaction between the two packages is proposed, but not yet scheduled.

So it is to be expected that things might not be working the way one wishes. However, the problems in the MWE can simply be solved by changing the package loading order:

\documentclass{article}

\usepackage[hang, flushmargin]{footmisc}
\usepackage[colorlinks=true]{hyperref}
\usepackage{footnotebackref}

\begin{document}
\tableofcontents            
\newpage                
\section{Section}
This is some text\footnote{This is a footnote.}.

\newpage\null% to see that the hyperlink works
\end{document}