Threewords.me is a site that helps "get three words about yourself" from your friends. It was made by Mark Bao and instantly went viral (exponentially grew). It was then quickly sold at private auction.
It is a super simple site that is inherently viral. People spread it themselves by sharing their unique links. You can see this process happening in real time by searching twitter and waiting to see new tweets load.
My advice to any new Internet entrepreneur is to clone this site immediately. Why? Because touching something truly viral is awesome.
First off, it's fun. Because the process is 24/7 there is always something going on to motivate you.
Second, you'll learn a ton in an accelerated fashion. Dealing with millions of users isn't easy, and you'll have to do it overnight. You'll get invaluable experience in scaling all aspects of a Web site.
Third, despite what people said about Mark's site, I think it will pay off quite well monetarily in the long run. If you can stick with it to ~10M users, I'm sure a lot of acquirers would be interested in you. Also, I think you'd be able to straight up monetize via virtual gifts, e.g. charge some facebook credits to send (virtual) thank you flowers/cards/sheep/whatever.
Fourth, you can use your user base to try to launch other startup ideas. Even if you can convert 1% of 10M users, that's a great initial base to launch with.
You don't have to clone threewords.me exactly, but that would probably work. Some easy tweaks would be rate which picture I look best in or which avatar most closely resembles me. There are many possible variations, some of which may spread even faster than the current implementation.
Even if you did clone it exactly, Facebook alone has 600M users, so you're not likely to overlap with the other clones that much if you start out in different parts of the network. I'm sure you could find interesting ways to seed it in more remote corners of The Social Network.
Describe me in three words: http://threewords.me/yegg