There's a great presentation about the issues of current online social networks (namely facebook): http://www.slideshare.net/padday/the-real-life-social-networ...

One key point he makes is that your social groups are usually disjoint sets of people and you may not want to share the same information across multiple groups. For example, you don't want your family to see pictures of you at a wild party that your college friend tagged you in.