I've been wrestling with this problem for a few years, I'm curious to see how Adaptive Blue's new Glue product tackles it. The problem focuses on how to adequately manage privacy concerns when user information is passively contributed to a community or social network.
In active contribution, it's pretty straightforward. I may actively bookmark an article on delicious, actively share a post on friendfeed, actively add a connection on LinkedIn, actively post my whereabouts on Twitter. The systems have different privacy options (keep private, restrict to your connections, make public, etc), but since this functionality is often difficult to understand, and often keeps changing (along with privacy policies) the general rule of thumb seems to be to only actively post what you comfortable being public.