From looking at the data provided by Rapleaf, women are much bigger users of social networks then men. Much bigger.

The fastest growing demographic on social networks are moms between 35 and 45 years old. These women are putting up pictures of their kids (Johnny at baseball practice, Susie at soccer, the family in Disneyworld) and using these social networks (especially MySpace) to essentially make family home pages and share them with friends and relatives. They are decorating their pages, making RockYou slide shows, and using lots of widgets.

Men over 35 are just not that into social networks (yet) except on some rare examples like LinkedIn.

While both younger women and younger men have both joined social networks in drives, the younger women are much more active than the younger men. My hypothesis is that a lot of the younger men are spending their spare computer time playing video games. Women tend to enjoy casual games more and most social networks are essentially a huge casual games network.