Don't forget FB Apps. The idea that social graph could be viewed as a data platform was revolutionary. I remember lot of young engineers around me going crazy about it and building silly apps. There were a lot of useful apps as well; book lending, used articles sales and so on.
Orkut, MySpace, Ning and other social networks had support for apps using OpenSocial around 2008. You had access to friends, FoFs, and several other social features.
For some time people ignored this platform, which meant that you could make a few million dollars per month porting successful FB games (Colheita Feliz, a FarmVille clone, comes to mind) since you faced virtually no competition.
