Facebook Inc. /zigman2/quotes/205064656/composite FB -0.34% shares fell 3% in after-hours trading Tuesday after a report that the Federal Trade Commission is preparing to file an antitrust lawsuit against the social-media giant. The Wall Street Journal reported Tuesday afternoon that the FTC is considering filing the suit by the end of the year, based on unnamed sources. The Journal reported that Facebook Chief Executive Mark Zuckerberg has testified to FTC staff in the investigation, which the Journal noted could still end without charges as the five-member commission has to vote on filing the lawsuit. Facebook has admitted in filings with the Securities and Exchange Commission that it is under investigation by the FTC and the Justice Department for potential antitrust charges. Facebook stock closed Tuesday with a 2.4% gain at $272.42, then ducked lower than $265 in after-hours trades following the release of the report.