Footnotes: distance between number and text

The issue with your code is that you put a \hspace after setting the \@thefnmark. You can fix this by changing LaTeX's version which is:

\newcommand\@makefntext[1]{%
  \parindent 1em%
  \noindent
  \hb@[email protected]{\hss\@makefnmark}#1}

A more LaTeX like approach using ifthen package would be:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{libertine,blindtext,ifthen}

\makeatletter
\renewcommand\@makefntext[1]{%
  \parindent 1em%
  \noindent
  \ifthenelse{\value{footnote}>99}{%
    \hb@xt@ 30\p@
  }{%
    \ifthenelse{\value{footnote}<9}{%
      \hb@xt@ 10\p@
    }{%
      \hb@xt@ 20\p@
    }%
  }%
  {\sffamily\bfseries\@thefnmark\hss}#1}
\makeatother

\begin{document}
Hello\footnote{I am a footnote!}\footnote{I am another footnote
  \blindtext}.
\addtocounter{footnote}{15}%
Hello\footnote{I am a footnote!}\footnote{I am another footnote
  \blindtext}.
\addtocounter{footnote}{99}%
Hello\footnote{I am a footnote!}\footnote{I am another footnote
  \blindtext}.
\end{document}

Adjust the length to suit yourself: I used 10pt as fixed distance and 0.5em for each digit.

This assumes the footnote mark expands to a positive number, of course.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{libertine}

\usepackage{expl3}

\makeatletter
\renewcommand\@makefntext[1]{%
  \proportionalnums{\bfseries
    \makebox[\getfnmarkwidth\@thefnmark][l]{%
      \proportionalnums{\@thefnmark}%
    }%
  }%
  #1%
}
\makeatother

\ExplSyntaxOn
\cs_new_protected:Nn \getfnmarkwidth:n
 {
  \dim_eval:n { 10pt + .5em * \fp_eval:n { floor(ln(#1)/ln(10),0) + 1 } }
 }
\cs_generate_variant:Nn \getfnmarkwidth:n { V }
\cs_set_eq:NN \getfnmarkwidth \getfnmarkwidth:V
\ExplSyntaxOff

\begin{document}
Hello\footnote{I am a footnote!}\footnote{I am another footnote.}

\setcounter{footnote}{9}

Hello\footnote{I am a footnote!}\footnote{I am another footnote.}
Hello\footnote{I am a footnote!}\footnote{I am another footnote.}

\setcounter{footnote}{99}
Hello\footnote{I am a footnote!}\footnote{I am another footnote.}
Hello\footnote{I am a footnote!}\footnote{I am another footnote.}

\end{document}

Explanation: floor(ln(#1)/ln(10),0)+1 is the number of digits of #1.