Kinda depends. Is the person joining these sites cause they "gotta get a GF/BF, just HAVE to get a GF/BF!", or cause they're like, "what the hell, maybe I'll meet someone, maybe I won't, who cares."? I'll admit, I've joined a few and usually unjoined once I discovered you have to pay. Although I guess I still have a OKcupid account and a plentyoffish account, but never really use them.
For the most part, I have to say dating sites are nothing but a waste of time and often money.