I don't have any data to back this up, but I wouldn't be surprised if many/most of those writers are tech writers, bloggers and similar. Google were smart about who was let in on the ground floor. 1) People who have strong social graphs (on the internet); 2) People who will actively engage and explore new technology. Sounds like a blogger to me.

The pro novelists I know have taken to social media in a strong way. Same with indie filmmakers.

Two reasons, I think. One, obviously, is the opportunity for promotion. But two, these are very lonely professions, and they seem to welcome any excuse for community.