Facebook’s self-serve ad program is younger and less tested than the AdWords program fueling Google’s growth.
It also doesn’t produce the click-through rates the search giant sees on ads running alongside its search results.

Ads need to be reviewed with click-through rates fall below 0.05 percent, says Facebook's Sarah Smith
Facebook Online Sales Operations Manager Sarah Smith declined to provide the range of click-through rates the social network sees on ads its customers buy at auction.
But don’t expect them to equal the rates Google gets with its flagship AdWords, she said Wednesday at the Search Engine Strategies conference.
The range varies widely, she added. But when they get to 0.05 or 0.04 percent, it is time to review or replace an ad.
Smith said successful campaigns can have click-through rates as low as that, but keyword prices need to be low as well.
Ads on Facebook also seem to degrade within two weeks, with click-through response falling. “I think two weeks is the max you want to run your ad” before changing the image it uses, she said.
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[...] “Facebook Says Click Through Rates Do Not Match Those At Google”. TechPulse 360. August 12, 2009. [...]