Facebook's stock, which already took a big hit last week, plunged as a result.
Facebook said in a statement on Monday that the company remains "strongly committed" to protecting people's information and that it welcomes the opportunity to answer the FTC's questions.
News outlets have reported on the FTC investigation last week, but the FTC hadn't confirmed it until Monday. Facebook reached a settlement with the FTC in 2011 offering privacy assurances.
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