GNOME released a census of the demographics of its open source contributors. (NB: Viewing a copy requires one to enter one’s email address.) The census reported that the top 40 individual developers contributed about 30% of the changes. Over 70% of contributors reported that they contributed on their own time, but paid developers (i.e. the remaining 30%) contributed 70% of commits.
The survey certainly bears out my own general experience and impression of open source development. While open source projects many have many contributors, most projects are primarily written by a small group of contributors. Also, while open source may be the mythical sandbox of hobbyists, in reality it is mostly the work of developers employed full time by technology companies. As to the stratification of paid versus unpaid developers, the report itself states that the figures may be inherently a bit inaccurate. I would add that based on my experience counseling clients, many paid developers would be likely to self-report that they are unpaid, when they are contributing “on their own time” – whatever that may mean in our type-A, Silicon Valley workscape.