My daughter (14) has a facebook account and I do monitor it closely. She's
really just yapping away with her girlfriends, but once in a while there is someone in her friend list she doesn't really know. It's a friends suggestion
or other recommendation and when pressed further, the friend doesn't really
know either. So these people get deleted right away, but I have to be the one
checking it in the first place.
I also monitor emails - if they're from someone she doesn't know, we read
it together. One time I wasn't home she got a somewhat suggestive spam mail
and she wrote back "don't write back to me you pervert!"