I'm in two:
The Crone Cronies group is very chatty and generates lots of messages every week. When I'm busy in real life, I post my workout and tell everyone to have a good day. When I have the time, I respond to those that there's something to say to. People in the check in pretty much post like they want. It's a wild and crazy place where people know each other, are very supportive, and where you never know what will come up. Right now, we're talking Droid phones and tablets (along with workouts, of course
).
Your Personal Best is sort of at the opposite end of the spectrum. The small group of people post when they can, sometimes drop out for awhile and come back. Since the stated goal of this check in is to support each other in our own personal goals, there's quite a bit of support there.
Whenever I try to add a third check in, I just can't do it. I end up dropping out of it and I feel really pressured for time. So, I guess 2 is my number. I don't lurk in check ins; I spend all my check-in time with my two groups.