Fling, AdultFriendFinder, FuckBook, and so on. Almost all of them offer a trial period where you can look but can't contact anyone, you'll see that you get private messages but can't read them. It's a pretty convincing scam, until you look closely at the Terms & Conditions. There will be a line buried down deep in there that says "some profiles are created and operated in-house that do not represent an actual person, and are used for the purpose of enhancing the user experience." Yeah, they basically admit that there are fake accounts.
And not a few, either. The vast majority of them are fake (well, the females at least). Even when you first load most of those sites and do their trial search/create a trial account, it's already set to "I am a: [MALE], Looking for a: [FEMALE]". It's a safe bet who's behind the keyboard. There are real women on the site, most of them are providers/sugar babies, or are so beastly that I wouldn't put my dick anywhere near them.
I tried one of them a few years ago, lured in with the fake private messages and deluge of "hot horny 20-somethings in my area". Paid for a 3 month membership and never even got a response from a real human being, let alone met any in person.
I got laid more from making random friends on MySpace. Not to say those paid sites are completely useless, but you have to put in a LOT of time to filter through the bullshit in hopes of finding that one girl who's half decent looking, not hoping for any financial support, and is REAL.