Are you talking about searching on Facebook or elsewhere? If you want to do Facebook groups, I can send you my Facebook account name, you can become friends with me and look at the >100 groups I'm a member of (went around for Round 1 (especially) of Chase Community Giving looking for big (100+) and then medium (e.g. 50+) groups). (We can un-friend each other when you're finished - generally I have 100+ friends requests I'm ignoring atm - I tried to set up another Facebook account just for ME/CFS last night and then Facebook flagged it that they're concerned I have two accounts.

)
There are lots of yahoogroups
www.yahoogroups.com also around the world with various activity levels.