I read this shallow but interesting article on Estadão today about social networks in Brazil:
Orkut ainda é rei absoluto, mas já dá sinais de cansaço (from Portuguese: Orkut is still king, but shows signs of weakening) - I wanted to point to the original source of the research by IBOPE, but I was unable to quickly find it on their website
What was interesting about this article is that 91% of the people that use social networks use Orkut in Brazil. Out of the people in Brazil that use social networks, only 14% use Facebook and 13% use Twitter. Unfortunately they don't show how many use both. My guess is that a good part of those people are "social network junkies" that will use all the ones they can, and generally they end up finding different uses for each of them. Another interesting number is that the average number of "friends" that people have in Brazil in social networks is 273.
I wished I had access to the raw data here to get more numbers, but I guess I don't really want to pay them to get this data. Oh, well...