Two of the biggest online dating sites (match.com and Yahoo!) are accused in separate lawsuits of defrauding members. Match.com is allegedly paying people to go on dates with fee-paying subscribers, which on the face of it sounds absurd. It's hard to see how you could make any money over the long term with that kind of business strategy.
Yahoo! is accused of posting fake profiles to make it look like there are more available "dates" registered in the service than there actually are.
It is important to remember we are still at the dawn of the Internet era. I'm less interested in the fraud, serious as the charges are, than in the "newbie" nature of many of these services. By that I mean I've always thought a lot of online "businesses" have prospered mainly because there are so many new people who have never tried them. So for a while, as long as you have more new people registering than drop off, you can have "growth." But eventually, you run out of people who don't know that online "dating" or whatever the service happens to be really does not deliver. And then the business goes downhill very quickly.
As Web 2.0 churns along, we'll hear more stories about these kinds of "thin" businesses, in the sense that their service is very thin--little to offer and too much reliance on gullibility or ignorance. That kind of business never lasts.