Most mergers valued at more than $66 million to the FTC, and preliminary reviews by the agency are common. But the investigation of Facebook for any competition issues could upset its plans to close the deal by the end of the second quarter, the FT reported.

Maxwell Blecher, an antitrust attorney with Blecher & Collins, said an antitrust probe would likely take at least a month.

"If they go into it further and they issue additional requests for information, that could probably take up to six months," he told IDG News Service.

The FTC has already collected information from at least two of Facebook's largest competitors, according to the FT report, which cited unnamed sources. Facebook will not be able to integrate Instagram's service with its social networking platform before the deal is finalized, the FT said.

That delay could hurt Facebook's to strengthen its mobile strategy. Facebook's has said in regulatory filings that its revenue will suffer as users increasingly access its service from mobile devices rather than PCs.