I think this is an incredibly interesting topic that has yet to be really deepened--for example, it's not just about how many friends/followers you have, but also about how much use you make of them. Having sample sizes of 300-400 people is enough (assuming random populations) to run some small statistical surveys about issues and ideas. A big way I've found to engage friends online is to post moderately interesting studies/articles/ideas from different areas on your own page. This allows for people who have those differing areas of interest to have a reason to look at them.

As far as gaining followers/friends, I don't think it's as much joining clubs and doing things as it is doing things that have as little overlap as possible. I have over 1000 friends (at least, according to Facebook!), but it's because my main areas of interest (skating, academia, gaming, theater) all have relatively small overlap. Contrast this to people I know with far fewer friends but work on more areas than myself.