The 4-1 vote leaves the final decision to the European Commission, which has launched its own inquiry into the deal.
"After carefully reviewing the evidence, we have concluded that Google's proposed acquisition of DoubleClick is unlikely to substantially lessen competition," the FTC majority said.
The commission also rejected concerns expressed by competitors, among them Microsoft, that the combination of the internet's No. 1 search engine with the No. 1 ad server would give Google overwhelming market power that could be used to drive away competition.
"Because the evidence failed to show that DoubleClick has market power in the third-party ad-serving markets, it is unlikely that Google could effectively foreclose competition in the related ad intermediation market following the acquisition," the FTC said in a press release accompanying the decision. "The evidence also showed ... that any aggregation of consumer and competitive data resulting is unlikely to harm competition in the ad intermediation market."
The FTC said the privacy concerns raised by the Electronic Privacy Information Center and the Center for Digital Democracy were outside the scope of its antitrust inquiry. (Separately, the agency today unveiled a major new proposal for a series of online privacy principles for websites, but suggested the principles should be adhered to voluntarily.)
One no vote
The one vote against came from Commissioner Pamela Jones Harbour, a Democrat, who questioned the conclusion that the deal wouldn't give Google market power.
"I dissent because I make alternate predictions about where this market is heading, and the transformative role the combined Google/DoubleClick will play if the proposed acquisition is consummated," she wrote.
Jonathan Leibowitz, the commission's other Democrat, while generally agreeing with the decision to let the deal proceed, said he was worried about "the substantial privacy issues" the deal raised and the possibility it could create vertical competition issues.