The gulf between the two social nets is dramatic. Advertisers will spend $1.28 billion globally this year to reach Facebook’s more than 500 million users, according to eMarketer. In contrast, MySpace, with its shrinking audience, is expected to draw just $297 million in ad revenues worldwide next year, down 14 percent from $347 million this year, the firm reports.

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While eMarketer has projected Facebook to have the lead in ad spending for a while, the latest numbers are a major upgrade from where the researcher’s forecast last year. In May ‘09, eMarketer predicted Facebook’s U.S. ad revenues would rise a healthy nine percent to $230 million last year.

In today’s release, eMarketer said that U.S. ad dollars going to Facebook in ‘09 were $500 million, while worldwide, it was $665 million. This year, eMarketer anticipates Facebook’s U.S. ad revenue to rise to $835 million—a 67 percent gain over ‘09.

Things will start to cool a bit next year, as eMarketer is calling for a 26.9 percent increase in U.S. ad spend on Facebook to $1.060 billion. Worldwide, that number will be $1.760 billion, or 37 percent higher than 2010’s expected take.

So why such a big change from just a year ago? It’s not the ad recovery, though that’s certainly helped. Rather, much of the reason for the bigger numbers is due to Facebook’s self-serve ad system, the tool advertisers use to create, buy and target the small ads that appear on the right side of most users’ pages, noted eMarketer analyst Debra Aho Williamson. The latest numbers appear to be in line with the optimistic view Reuters (NYSE: TRI) found in June.

The self-serve function probably accounts for at least half of Facebook’s revenue, she said, adding that previous forecasts did not fully account for the size of this business.

To put Facebook’s success in context, its ad revenues have not only eclipsed MySpace’s, but it appears to be closing in on the $890 million in U.S. ad revenues eMarketer believes AOL (NYSE: AOL) will reach this year.