People use services in very different ways, and have very different views on what is acceptable privacy-wise. Personally, I struggle with the "all-in" nature of Facebook (and Buzz, but there's no social pressure to use it yet) - I have various social circles offline and online that simply don't overlap (e.g. family, friends from school, friends from university, friends in the various towns/countries I've lived, customers/business partners, etc.), and the tone, topic and nature of communication with each group is different. I'm not talking about "secrets", I just don't think it's appropriate for them to be forcibly stirred into a pot, and people don't need to know about everything without even trying to dig it up.

I even resent the public friends list on Facebook, but at least I get to decide who lands there (unlike the initial Buzz rollout). As a result, I don't use Facebook actively and just add people as friends because it's perceived to be rude if I don't. I've noticed I'm not the only one - there seems to be a pattern: someone discovers Facebook (note that it's only recently become popular in Austria; it was practically unknown outside the web scene in 2008), get excited and add all their friends, post pictures, etc. and after a while remove a lot of personal details, delete photos and only participate actively by replying to others. Maybe this has already played out a few years earlier in the US.

Oddly enough, I really enjoy Twitter. I guess it's because it's very up-front about everything being public, so the temptation to post something you might regret later is reduced.