I sold a reasonably popular (1MM searches a month at peak time) for a very small amount of money. It was a Bebo search engine, and as its popularity followed the trajectory of Bebo (except AOL didn't come knocking with 850,000,000 sheets).
My main takeaway is that if you're building a business that depends entirely on another service (whether it's Bebo, twitter, Facebook, iPhones whatever), you're at the mercy of someone elses business decisions. If you're planning on building something valuable to last a long time, then that's the wrong place to start.
I wrote about the story here and extracted a few lessons http://destraynor.com/serendipity/index.php?/archives/179-Le... It's a good read (though I would say), I'm not pimping for traffic here, just letting you all know.
My main takeaway is that if you're building a business that depends entirely on another service (whether it's Bebo, twitter, Facebook, iPhones whatever), you're at the mercy of someone elses business decisions. If you're planning on building something valuable to last a long time, then that's the wrong place to start.
HTH