It's 2012! Mercurial and Git are both still strong.
I understand the trade-offs of both. I also understand everyone has some sort of preference for one or the other. That's fine.
I'm looking for some information on level of usage of both. For example, on stackoverflow.com, searching for Git gets you 12000 hits, Mercurial gets you 3000. Google Trends says it's 1.9:1.0 for Git.
What other empirical information is available to estimate the relative usage of both tools?
In a similar style to my Git vs. SVN answer, Ohloh has been crawled (only) three times by the Internet Archive's Wayback Machine, but July 2011 is unreadable:
Another source of data is the Eclipse Community Survey. Git values below are for Git/GitHub.
These appear to show that, of the open source repositories registered on Ohloh, and of the developers using Eclipse, Git is a good order of magnitude more popular than Mercurial.
I think other than Google trends or SO questions (which as the comments above point out, might indicate curiosity or difficulty rather than usage), your best bet is to look at statistics where they are available, and weight them by source (how you do that is likely suggestive, though).
You can see the distribution of ALL version control systems on projects indexed with Ohloh.
Debian Popularity Contest shows a graph for stats for DVCS packages.
And it's a little outdated, but the GNOME DVCS Survey Results are interesting.
When it comes down to the numbers, I think Ohloh is the most general audience, so I'd go with that, personally... still a LOT of people using SVN and even CVS, though.
In terms of open source software, where the important issues are coordinating broadly distributed and asynchronous teams, Git is the hands-down winner. Especially when you look at Wikipedia's comparison by popularity of open-source project hosting sites (based on numbers of GitHub (git) vs. BitBucket (Hg)).
