According to the latest data from Hitwise, Facebook, which is kicking off its developer conference today, grew 40% over the last year, while its biggest competitor, MySpace, saw a 6% drop in visits since June 2007. The numbers for average time spent on these two sites almost directly mirror the numbers for visits, with MySpace being down 4% and Facebook up 41%.

Even though Facebook is slowly catching up to MySpace, the gap between the two is still large, with Facebook having a 17% market share and MySpace 72%.

While Facebook users are starting to spend a lot more time on the site, the average MySpace users still spends about 10 more minutes there than the average Facebook user (21 min).

MyYearbook, interestingly, saw the largest gain of all social networks, a trend we have observed for quite a while now. It grew by almost 320%, though it only has a market share of 1.5%. Its users are also among the most loyal and spend about 30 minutes on the site. MyYearbook, as the name implies, mainly targets the high school market.

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