I see "private" Social Groups and Forums as nothing more than an expanded version of Private Messaging.

I never know what other people are PMing each other about unless they happen to include me somehow. I assume they have lots of things to talk about besides me and if they ever do mention me, it's nobody's business but theirs if they choose to say nice things, not-so-nice things, or things that happen to mean something to them but would mostly just confuse me if I were to see them.

If I were ever in the mood to be paranoid I'm sure I could imagine dozens or hundreds of other members PMing each other that they don't like me because, oh, I eat too much garlic -- even though as far as I know, no one from PC has ever been close enough to me to know when I last ate garlic.

Mostly, though, I'm relieved that if people ever do want to talk about me, they can do it privately where it's not my responsibility to respond to, or even know, everything they say.

One of the best reasons I can think of for keeping Social Groups private or semi-private, is to provide a place to discuss religious or political subjects away from those who'd be upset by such discussion and/or feel compelled to wade in and chastise the participants.