Shared in GTK+ Healthcheck from his blog, Alberto created some charts that show the number of unique contributors working towards each GTK+ release. In addition to contributors on the overall code-base, he also plotted the number of contributors working on translations each release.
Alberto's data basically shows more people than ever are contributing to the GTK+ project. It's a rough average about 60 people working on each GTK+ release. "There are a few points in time where contributions have risen up noticeably, one is in the 2.18 release, which I suspect has to do with the adoption of Subversion over CVS, and the other one is during the 3.0 release."
Alberto also says, "Not only in terms of people contributing patches, but 3.x has been an outstanding release series (though a bit bumpy stability wise), CSS theming and a new cairo based theming API, the broadway and wayland backends, file and font chooser improvements, a massive code cleanup in several places... the list just goes on. Not to mention that the grounds for 4.0 are being settled already like the paint clock work by Owen and the work Emmanuele has been doing on Clutter 2.0...So all in all, I think that there are challenges, but we have loads opportunities to improve as well. We have many facts to celebrate and be excited about."
On a similar note, be sure to check out the 2012 GNOME User Survey results.