Although praise has been heaped on Facebook for opening its API to developers and providing a seemingly more valuable audience to advertisers, the social network still has less than half the traffic of MySpace--drawing in more than 19.1 million unique visitors in August, compared to the latter's more than 60 million, according to Nielsen.

While News Corp.'s social network has been a consistent source of impressions for marketers (with traffic up 23% from the same time last year), Facebook pummeled MySpace when it came to growth--increasing traffic by 117% year-over-year.

Those traffic numbers should continue to move higher--as Facebook recently made its user profiles more readily available to search engines, and its users are also particularly fond of page-view-driving widgets--although the question remains as to whether the influx of attention will degrade the privacy mechanisms that have made the social network so popular.

Classmates Online--a social network aimed at reuniting high school and college friends--took the third spot in Nielsen's ranking, with more than 15 million unique visitors. Meanwhile, music and pop culture-focused community Buzznet.com and Disney's Club Penguin showed phenomenal growth--with traffic surges of 405% and 250%, respectively.

Google's YouTube reigned supreme in online video--nabbing more than 56 million unique visitors--a growth spike of 66% from last August. MySpace videos took the second spot with over 16 million uniques--but the social network was the only site that lost traction (-6% year-over-year) when it came to viewers. Non-YouTube Google video sites rounded out the top three.

But the ones to watch in the video space are Veoh and Yahoo--which increased traffic by 346% and 101%, respectively. Advertisers seeking alternatives to media outlets like NBC or Disney for video may want to keep an eye on these two properties--as they may be offering impressions at a more attractive cost.

Meanwhile, YouTube's announcement that it would begin overlaying ads on some of its videos in late August led to a bit of user backlash--next month's report will show whether it translated to any real drop in visits.

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