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U.S. Justice Department to open an antitrust investigation of Facebook -- source

WASHINGTON, Sept 25 (Reuters) - The U.S. Justice Department will open an antitrust investigation of Facebook, a source familiar with the matter said on Wednesday.

It will mark the fourth investigation of the social media company which also faces probes by the Federal Trade Commission, a group of state attorneys general led by New York and the House of Representatives Judiciary Committee.

(Reporting by Diane Bartz Editing by Shri Navaratnam)