Check out 7 Reasons Why Web Apps Fail, a great article by Josh Porter on Bokardo.com. Here are the main points:

1. Focus on social instead of personal.
2. They solve too many problems, or try to.
3. They’re about making someone other than the user happy.
4. They sell it the wrong way.
5. Not in it for the long haul.
6. They show too much of what’s going on, and get gamed.
7. They don’t have an underlying business strategy of improving people’s lives.