|Yeah, that's all true... although the paranoia about RL details escapes me...|
Examples of the problem from people I know:
Supposing you had an American friend on facebook, and you were on some forum talking about how you disagreed with American foreign policy, and you said some things that, while true, were put a lot more bluntly than they would be if you were talking to your American friend - and your American friend looks on your profile, sees the link to the conversation, and decides you hate Americans and you get a weird reputation among your friends as an American-hater.
Supposing you've just started working in an office full of temps, you join the facebook network for it and you're expected to add others on facebook as friends. Without knowing much about you they see some sort of political stuff - group memberships, forum posts - which they misinterpret, and then you're whispered about as being a terrorist sympathiser, etc. Now admittedly, in this case the person was a member of some pro-Palestine groups, and the people accusing her of being a terrorist sympathiser were un-controversially wrong, that didn't stop it being a pain at work.
That sort of thing.