A big problem with many of them is the large number of 'fake' profiles people put up.
I don't mean people that are dishonest about this or that, but there are actually people that make this fake profile with fake pictures saying something like "female, 35, Seattle" and they're really just some Russian looking for a ticket into the US, or just someone playing a joke. There was one site I read about a while back where something like 55% of the female profiles were like that. It was a fairly popular one, too.
There's a really high percentage of married men on these sites as well. Between the internet and individual cell phones, it's become incredibly easy to have affairs.