Cleaning up my office the other day, I found an interesting chart comparing traffic numbers for top social sites year-to-year between November 2005 and November 2006.

Xanga was number two among the 12-to-17 crowd, increasing their audience 29 percent (to 2.1 million monthly unique visitors). Facebook was sliding, down nine percent with the young ‘uns (to 1.5 million), but up 14 percent among 18-to-24-year-olds (to 5.5 million). MySpace? Tops in both age groups, up nine percent with the youngsters (to 6.4 million) and up 34 percent with the older group (to 6.8 million).

Which led me to wondering how those sites are doing today, which led me to this TechCrunch page of social site rankings for October 2007. From 2006 to 2007, Xanga traffic slipped 49 percent, MySpace increased 28 percent, and Facebook exploded, increasing 118 percent. Digg and LinkedIn didn’t too too shabbily, either, increasing 280 and 257 percent, respectively.

Takeaway for marketers: Diving into the social networking waters? If so, remember that audiences are notoriously fickle. Those top 50 social networking sites listed over here? Well, here’s another 300 looking to eat their lunch … and there’s undoubtedly plenty more where those came from.