URL of cited web site in bibliography

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.

I checked the file chicago.bst on my TeX system (MacTeX 2011): its file date is "28 August 1992" and its abstract notes that it is a "BibTeX bibliography style that follows the 'B' reference style of the 13th Edition of the Chicago Manual of Style". Given that this style file was created in 1992, it's probably not surprising that it does not recognize (let alone process) the "URL" field of a bib entry. Hence, even if your bib entries have url fields, they are not going to be printed out if you use this style file.

A real solution would be to find a (much more!) recent style file that implements the recommendations of the current, i.e., 16th edition of "Chicago". Off the top of my head, I'm not aware of such a file, but hopefully other participants in this group can chime in and provide advice. A temporary workaround (i.e., until you manage to find such a .bst file) is to change the field type from "url" to "note" of your bib entries. If you choose to pursue this route, you will also need to encase the URLs in the command \url{...} and load the url package with the hyphens option, i.e., \usepackage[hyphens]{url}. Taking your example, you'd rewrite it as:

@misc{Carlson2011, 
  author = {Carlson, Nicholas},
  booktitle = {Businessinsider.com},
  title = {{How Many Users Does Twitter REALLY Have?}},
  note = {Available online at \url{www.businessinsider.com/chart-of-the-day-how-many-users-does-twitter-really-have-2011-31/3}},
  year = {2011}
}

Alternatively, you could consider switching from bibtex/natbib to biblatex and loading the package biblatex-chicago. See the question bibtex vs. biber and biblatex vs. natbib for some guidance on how to achieve the (not too onerous) transition from bibtex/natbib to biblatex.

If you are willing to switch to biblatex (see bibtex vs. biber and biblatex vs. natbib for the many advantages of biblatex), you could use its very complete implementation of the chicago style, biblatex-chicago. It provides an online entry type, which is specifically tailored for online publications / websites (url, date accessed, etc.).

For the @online entry type see also: https://tex.stackexchange.com/a/3608/120395