Wondering where to spend your time optimizing for social media? MySpace is still the outright leader of all things social media, accounting for 76% of all social networking visits among a category of 53 of the top social sites in 2007, according to new Hitwise data. Facebook followed with 12.57%, then Bebo with 1.24%. BlackPlanet.com and Club Penguin rounded out the top five. As far as growth, Facebook saw a jump of 51% in December 2007 from a year ago, while MySpace dropped 8%.

A couple of other interesting points:

• In December 2007, MySpace received 95% of its traffic from return visitors (defined as a repeat visit within 30 days). Facebook saw 93% of its traffic from return visitors.
• New visitors accounted for 5% of MySpace traffic and 7% of Facebook traffic. Bebo saw 16% of its traffic from new visitors.
• U.S. visitors spent an average of 29:36 on MySpace in December 2007, a change of -0.2% from last year. Facebook visitors spend an average of 19:51 on the site, an increase of 100% from last year. That's telling, and no doubt influenced by Facebook's decision to open the site to third-party developers.

Finally, the report makes quick mention of an interesting statistic: U.S. traffic to all the social networking websites increased four percent year-over-year. With all the buzz over social networking and media and how it's changing the Internet and changing the rules of marketing, business and SEO ... four percent just doesn't sound like much.