I posted about these in another forum, but This
Report makes interesting reading for parents.
I know my own kids use these networks (as do I - and I'm friended by most of their friends, which is kinda cool).
But if you don't know what they are, it's worth checking them out, if you've got kids with computer access! This isn't to scaremonger - the networking sites are useful means of communication.
But like any other tool, it needs to be used properly... Leaving a child alone with any power tool is a dangerous thing.
(For more, slightly older information - A Review of Risk)
Although my children all grew up using a computer, it was in the days before "the 'net" so I didn't have these issues. I did, however, always have the computer set up in a "public" area of the household so that one of us could offer assistance as needed, and, to a lesser degree, keep an eye on what was happening. As the internet grew, this became even more so. Now, especially in homes with younger children, I recommend the "public" computer even more so.