if u go to the topics section, you'll c how many people are following each topic. that gives u a rough idea. u gotta remember that most will overlap though. based on the GaG community topic, imma take a stab and say about 620, 000.
There are a couple of people in my followers list who are following about 4000-5000 people... so, well, at least about that many. (:
I think it's about the right size community for non-specialist questions. I like to use reddit for anything that's more obscure or specialized, but, the problem with asking general questions on reddit is that you get 15,000 responses. lol