Facebook has drubbed MySpace on almost all fronts—in the media, with users, in growth, in traffic—except ad revenue. But expect that to change next year, according to eMarketer: “It will surpass its former rival, MySpace, in ad revenues in 2010, when marketers worldwide will spend $605 million on Facebook versus $385 million on MySpace.”

The projections for next year show MySpace on a downward trend, falling from $490M worldwide this year to $385M next year. Facebook on the other hand is still climbing: from from $435M this year to over $600M next year.

Meanwhile, overall social network ad spending is going up. eMarketer predicts 7.1% growth for total ad spend next year, bringing the total to almost $1.3B. Although they initially expected 2009 to see a downturn in revenue, now the stats show 3.9% growth over last year.

Back in September, we saw that 20% of all online advertising was on social networks, with MySpace slightly leading Facebook (9.2% to 8.2%). I suppose we can expect the social share of online advertising to continue to grow—but not MySpace’s.

The biggest factor contributing to Facebook’s revenue growth this year, according to ClickZ, is its growth. They reached 200M users in April—and just five months later, they’d added another 100M users. Now at 350M active users, Facebook has doubled in size since February of this year.

Yeah, I’d say that would drive some revenue growth.

What do you think? Will Facebook really pass MySpace? How has MySpace been able to retain its revenue lead for so long?