One thing that I've noted in recent decades is that there is an overall decline in community organizations overall, such as Kiwanis, Rotary Club, and other such social and service organizations. Church membership appears to be in decline as well. There are some atheists who have organized, but that doesn't mean that most atheists would join up. Even here on RF, there's very little activity in the Atheism DIR forum.
I'm not sure why that is, although when you think about it, I guess there isn't a heck of a lot to talk about among atheists. There is no "god" to talk about. There might be a certain degree of political alignment on some issues, but it's not something where there would appear to be any need for a weekly meeting. Some religions might need to hold some sort of weekly gathering to proclaim and reaffirm their belief, but atheists don't really need to do that. What's the point of going to a weekly meeting to proclaim one's non-belief?
I might attend the Church of the Flying Spaghetti Monster, but only because there's spaghetti.