In the first half of the last decade, there seemed to be highly motivated, talented groups of contributors who helped build and maintain momentum in many popular open source projects like Gnome. I'm not saying there aren't top notch developers still working on FOSS, but many of those big-name 'rockstar' contributors seem to have either been lured to a competing/alternative platform, or just simply lost interest.
I often wonder if age plays an important role, with many former wide-eyed, young, idealistic developers possibly adopting an attitude along the lines of "meh. Maybe I should just buy a Mac and worry less about ideals and philosophies" as they get older and priorities change.
Many have been hired by companies, e.g. Nokia, and now don't want to work for free again.
Now if we live in a Resource Based Economy, then that is possible, but until then, money is king.
I contribute to GNOME, and that is not "working for free". It means having a fun hobby. If I'd get money for it, it would no longer be a hobby, but work. I did hear some people say that their hobby is the same as their work. IMO, work is more restricted (try doing nothing at all for 3 months with your hobby, then repeat for your work).
The typical open source developer has always been either (1) gainfully employed at a programming day job, (2) employed to actually build FOSS, or (3) in academia.
There's at least as much open source development going on now as ever before. It's just that the attention has shifted away from desktop apps.
http://gs.statcounter.com/#os-ww-monthly-200807-201207
http://www.w3schools.com/browsers/browsers_os.asp
Of course this does not include Android, or the numbers would look very different.