Facebook has had considerable success in 2008, they've grown consistently and the Compete.com data shows their growth trajectory to be significantly higher than MySpace.

I think one of the real successes of Facebook is that they have innovated quite a lot over the last year. They changed their user interface and have been adding a fair number of new features to their offering. This is really essential given the rapid growth in members they've had. As you grow your membership and user base changes. You need to change with them.

But Twitter is the real success story of 2008 with incredible growth catapulting them to the 3rd most visited social network.

I've written a bit more about this (and what Twitter can learn from Facebook's growth)