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).