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ReadWriteWeb says:

Online reputation company Rapleaf has released a new study of 49.3 million people, revealing gender and age data about social network users. On most of the main social networks – including MySpace, Facebook, Bebo, Hi5 – women outnumber men by a considerable amount. On Facebook, the 18-24 age group is largest, with 1,685,029 women in that age group compared to 977,753 men. In MySpace, the same age group dominates, with 7,091,214 women and 5,226,788 men.

The only social networks studied that didn’t have more women than men in the 18-24 year old group were venerable old LinkedIn (where incidentally the 25-34 age group was tops) and a site called Perfspot.

See the complete article for a chart of various analysis of social networking users.