With its more than half billion users and privately-held status, Facebook's revenue has long been a favorite guessing game for observers from Silicon Valley to Wall Street. But a new estimate from eMarketer says the company will book $1.285 billion in global advertising alone this year, almost double the estimated $665 million the company took in last year. That figure doesn't include Facebook's so-called virtual currency trade, which would nonetheless account for a fraction of the company's overall business.

Interestingly, Facebook's fastest growing area comes from its self-serve ad platform, which launched in August 2007. "We believe it accounted for about half of all ad spending on Facebook," eMarketer senior analyst Debra Aho Williamson said. "It's really become a tremendous business for the company. We didn't account for the size of that business last year in our estimate, but we found that it's become a great tool for direct-marketing advertisers."

The eMarketer analysis stands as a significant third-party verification against some of the provisional numbers floating around in the media. A July Bloomberg article, for example, cited two anonymous sources indicating Facebook would book $1.4 billion this year. A March report from the Wall Street Journal pegged the company's 2010 revenue at a wide range of $1.2 billion to $2 billion, also citing anonymous sources. In both articles, it was unclear whether the information came from inside or outside the company itself.

Like Google, Facebook's self-serve advertisers are largely local, coming from marketing dollars once spent on yellow pages listings. If those advertisers perform anything like Google's, they won't be as cyclical as national brand advertising, giving Facebook a buffer against future ad slumps.

Naturally, the other half of Facebook's revenue comes from display advertising, which the company recently ramped up after Microsoft's deal to sell Facebook's banner advertising ended in February. The site now serves all its own uniquely sized units; they are smaller and come at a lower price point. That may also explain, despite their hefty revenue, the very low average ad rate of 56 cents for every 1,000 impressions Facebook brings in, compared to the $2.43 average for the internet at large.

In part driving those bottom-dollar ad rates is the fact that Facebook accounts for a significant percentage of all display ad impressions on the web -- 16.8% of all impressions the U.S. in May, according to ComScore. That makes Facebook responsible for almost a fifth of all ad traffic in the country.

While Facebook is ramping up ad revenue, MySpace can barely hang on to the budgets that remain. The News Corp.-owned social site stands to bring in $347 million this year, a 26% drop from the estimated $470 million from last year, according to eMarketer. Further, its going to get worse before it gets better: EMarketer estimates MySpace will come in under $300 million in 2011. The ailing company recently hired ad agency Pereira & O'Dell for its first consumer brand campaign this fall.