Let’s get the bad news out of the way.

As a result of the poor economy and various difficulties at MySpace, paid advertising on online social networks in the US is expected to fall 3% in 2009.

But the drop will be short-lived.

eMarketer projects that US marketers will increase their social network ad spending 13.2% in 2010, to $1.3 billion.

“The expected rebound in spending will come as more companies focus on creating and implementing an overall social marketing strategy,” says Debra Aho Williamson, eMarketer senior analyst and author of the new report,Social Network Ad Spending: A Brighter Outlook Next Year. “And it is a clear indication that the experimental phase of social network marketing is finally drawing to an end.”

2009 is turning into a year of major shifts in the social network business.

“Facebook, once a distant second to MySpace, has outperformed its rival in nearly every measure of usage—and is on track to surpass MySpace in ad spending by 2011,” says Ms. Williamson.

US spending at MySpace is expected to fall 15% in 2009, to $495 million, while US spending at Facebook is projected to rise 9%, to $230 million. Consequently, MySpace’s share of US spending is projected to fall to 43.4% in 2009, while Facebook and other social network venues will increase their share.