FTC commissioners voted unanimously to close the agency's investigation into deal, once valued at $1 billion in cash and stock, the agency said in a statement. With Facebook's stock price at $19 a share, half of what it was when it went public, the deal is now worth about $747.1 million.
"We are pleased that the Federal Trade Commission has cleared the transaction after its careful and thorough review," a Facebook spokesperson wrote in an e-mail. Instagram representatives did not immediately respond to a request for comment.
The plan got regulatory approval in the U.K. last week.
Instagram is one of, if not the, fastest growing mobile apps in history, surpassing 80 million users in less than two years. Facebook's surprise move to acquire the hot app illustrated Facebook founder Mark Zuckerberg's urgency to improve and expand the social network's mobile strategy.
Updated 3:20 p.m. PT with more background information.