Vincenzo Cosenza maps social network dominance around the world according to traffic data from Alexa and Google Trends. We see Facebook has apparently overtaken MySpace in the US along with other countries; Orkut is a favorite in Brazil; the people love QQ in China; and then there are a few smaller networks that most of us have probably never heard of unless we live in the country of dominance.
I personally don't know anyone who uses anything other than Facebook or LinkedIn. Remember Friendster? People always laugh when I mention it. What do you use?