Here are some surprising findings:Facebook still skews young, but the 45- to 54-year-old age bracket has seen 45% growth since year-end 2012. Among U.S. Internet users, 73% with incomes above $75,000 are on Facebook (compared to 17% who are on Twitter). Eight-six percent of Facebook's users are outside the U.S.Instagram: Sixty-eight percent of Instagram's users are women.Twitter has a surprisingly young user population for a large social network — 27% of 18 to 29-year-olds in the U.S. use Twitter, compared to only 16% of people in their thirties and forties. LinkedIn is international and skews toward male users. Google+ is the most male-oriented of the major social networks. It's 70% male.Pinterest is dominated by tablet users. And, according to Nielsen data, 84% of U.S. Pinterest users are women.Tumblr is strong with teens and young adults interested in self-expression, but only 8% of U.S. Internet users with incomes above $75,000 use Tumblr.