I think our story, although not Facebook-famous, is interesting because I believe we significantly improved the web without anybody knowing it.
We started Defensio when we found out that many people were dissatisfied with the only anti-comment-spam solution that then existed: Akismet (owned by Automattic/Wordpress). Your favorites bloggers such as Scoble were complaining that Akismet was blocking way too many legitimate comments and that he simply couldn't go through thousands of blocked comments a day to find the good ones. Other famous bloggers such as Marc Andreessen (if I remember correctly, I might be wrong) were also complaining and about to shut down comments on their blogs. We thought that was terrible and we wanted to fix that. And we did.
We basically built a product that was more efficient and easier to use than Akismet. Although we certainly didn't put Akismet out of business or grow anywhere near as big as they are, we forced them to improve their product in order to stay in the game. Later on, a 3rd contender joined the race: Mollom.
I believe that in many ways, we made the web a better place. In part because of us, user-generated content still flows freely on the web, you can still comment without too much hassle on your favorite blogs. Competition is good, and we created that in a stagnant but very critical market.
By the way, it wasn't easy. It was hell at times. But we didn't give up even when it made absolutely no sense to keep going. That's why we succeeded.