Some of the recent incidents in the news where people were reprimanded or even let go from jobs due to their social networking postings often weren't taking the precautions discussed here again & again.
I wouldn't dare post something like "woo hoo I pulled a sick day to go to the salon.." as an example. I also am careful about the pictures of myself I share, no matter if they are of me alone, or with other people.
I would recommend if there are people who really pester you about your social networking site memberships, you can accept them if you want to but its not rude to keep your office life and your home life separate. I know people that set up 1 account for one, and 1 for the other to avoid issues.