The style chicago provides the following entries:
address author booktitle chapter edition editor howpublished institution journal key month note number organization pages publisher school series title type volume year
You see there is no entry url. To use one you can use the entry note:
note={\url{www.businessinsider.com/chart-of-the-day-how-many-users-does-twitter-really-have-2011-31/3}}
EDIT
To find which entries will be supported by a bibliography style you can open the relevant bibliography file. The extension of such a file is bst. To find the file on your computer use the command kpsewhich:
marco@marco-linux:~$ kpsewhich chicago.bst /usr/local/texlive/2011/texmf-dist/bibtex/bst/chicago/chicago.bst
To open the file via the terminal you can use in Ubuntu:
gedit `kpsewhich chicago.bst`
on a Mac:
open `kpsewhich chicago.bst`
In the file you find all provided entries in the following structure at the beginning of the file:
ENTRY { address author ... } {}
An example:
\documentclass{article} \usepackage{filecontents} \begin{filecontents}{\jobname.bib} @misc{Carlson2011, author = {Carlson, Nicholas}, booktitle = {Businessinsider.com}, title = {{How Many Users Does Twitter REALLY Have?}}, note = {\url{www.businessinsider.com/chart-of-the-day-how-many-users-does-twitter-really-have-2011-31/3}}, year = {2011} } \end{filecontents} \usepackage{natbib} \usepackage{hyperref} \begin{document} \cite{Carlson2011} \bibliography{\jobname} \bibliographystyle{chicago} \end{document}
The example uses the package hyperref to provide the command \url. Instead you can use the package url.
minimum working example(MWE). In the present case, it would be especially helfpul if you provided a specific example of a bib entry that has a URL you're trying to get typeset but (as of now) is not.