Shares of Facebook (FB) - Get Report slid Monday, closing down 7.51% after the Federal Trade Commission reportedly won jurisdiction over the company, giving it the green light to begin a potential investigation into the social media giant's competitive practices - and whether they are monopolistic and unlawful.

Facebook stock sank to $164.15 on the Nasdaq Stock Market after The Wall Street Journal reported that the FTC said it had struck a deal with the U.S. Department of Justice, giving it the go-ahead to focus on Facebook and for the DOJ to take the lead in a probe of Alphabet's Google (GOOGL) - Get Report  .

The FTC and DOJ share authority to enforce U.S. antitrust law, and at times must work out jurisdictional authority on which agency handles what issues.

Shares of Alphabet, which runs Google, also sank some 7%, or $77.35, to $1,026.28 in afternoon trading on the Nasdaq.

The U.S. Department of Justice was already gearing up for an investigation of Google, according to a published report late Friday. The probe will examine the company's practices in search and other businesses, the Journal reported, citing unnamed people familiar with the matter.

Google was fined $1.7 billion earlier this year by European regulators for antitrust violations in the online advertising market.

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The FTC, meanwhile, has already been ramping up its scrutiny of the tech giants in recent months. In February the agency announced a task force to examine competition issues it believes are issues in the world of technology service providers like Facebook.

FTC officials have said that, among other things, the task force would re-evaluate past government decisions that allowed tech giants to acquire smaller firms that potentially could have been future competitors.

Among the prior deals on the FTC's radar screen are Facebook's acquisitions of WhatsApp and Instagram, people familiar with the matter told the Journal.

The FTC's actions come amid calls by consumer advocates and some politicians for large tech companies like Facebook to be broken up.

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