Nielsen have release some year on year figures for visitors to the Top 10 Social Networks (based on US traffic so Bebo doesn't figure in case your wondering).
Facebooks' growth is clear but the interesting one for me is Linkedin. It's a smart tool that's been around for sometime but the combination of some smart features over the last year and the halo effect from mainstream adoption of social networks has given them a whopping 245% increase on 2006. They will be an interesting one to watch over the next few months, particularly to see what happens when they make their API available.
