Meeting someone online can be the flipside of meeting someone in real life. What I mean is, you know that situation in real life where you see someone who makes your blood race, but then on that first date, you wonder how the hell you can get out without hurting her/his feelings? Online is often the opposite. The rap is the thing. So someone can get your juices going if they communicate well, but then you meet them and the physical thing is about as hot as it is with your 80 year old Uncle Bernie.
OTOH, I lived with someone for about seven years that I met online. We're still friends and call each other once a month. So there's nothing weird about it.
I think meeting in chat rooms and placing ads are different experiences also. I don't know anything about specific dating sites. I once placed an ad and had to wade through so much absurdity that I was truly grateful that I'd gotten a garbage email account. Guys that tried to impress me by telling me all about their expensive toys, guys that had to make it clear that even though they were in their 60s, they could still get it up (in an introductory email, really?), guys that wanted to find the love of their life to take out of the city and to their home in the middle of freakin' nowhere... But I got a few fun dates out of it, so it wasn't a total waste. I wasn't looking for anything serious - which anyone with a lick of sense would have known from the ad.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine