I was in that class, and wrote an app that got 1 million users. That sounds good, but honestly, it was a mistake on Facebook's part. They made it much too easy for people to spam their friends. The most successful apps in the class tended to be ones where you just sent a "hug" or a "kiss" to your friends, that kind of thing. The lessons of the class (optimizing a viral loop) are still useful, but it pains me to see anyone overly impressed by it.