Flixster is just another social network, except that they are not.
Most social networks focus on you, while Flixster revolves around movies. A lot of people love movies. Just check how many people go to RottenTomatoes and IMDB.
I have no idea how they did it, but in less than 1 year they have millions of users. Their growth is closer to YouTube growth more than any other company I seen before.
But...
This is a niche/vertical play. They can even add blog, and photo albums and other things to compete more with MySpace and Facebook, but they are branding themselves as a social network for movie buffs, and that has a lower ceiling.
I won't matter much because they will be bought soon by some studio or major media outlet.
By the way, their content is a bit addictive for the first 30 minutes. Can't talk about more than that because I wouldn't sign up for such a service.