In building a possible antitrust case against Facebook, regulators are focused on its dealings with smaller rivals.

The Federal Trade Commission has been interviewing a number of current and former competitors of Facebook (FB) - Get Report , including tech executives and developers, the WSJ wrote on Monday. And Snap (SNAP) - Get Report , which reportedly kept a years-long dossier tracking alleged anticompetitive conduct by Facebook, could have a particularly compelling story to tell federal regulators. Facebook shares were down 1.46% on Monday to $187.16.

According to the report, Snap officials documented the various ways in which it believed Facebook was attempting to drive it out of the market as Snap gained popularity among younger users. According to those files, which were dubbed 'Project Voldemort' internally at Snap, the aggressive tactics kicked into gear after Snap CEO Evan Spiegel spurned an acquisition offer by Facebook CEO Mark Zuckerberg in 2013. Facebook's tactics included a prohibition on adding links to Snap profiles within Facebook's Instagram app, tracking Snap usage through Ovano, a Facebook-owned mobile analytics app that it has since discontinued, and perhaps most infamously, copying a series of popular Snap features, such as lenses, filters and stories.

Partly due to competition from Instagram, Snap's user growth stagnated in 2018 and its share price steadily plummeted, though it has bounced back 194% to $17.12 per share this year.

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Another area of focus is Facebook's habit of buying out smaller tech firms. Facebook has made more than 70 acquisitions in its lifetime. Two of those, WhatsApp and Instagram, wound up integrated into Facebook's core group of social apps, but in many cases the acquired company was shuttered in short order.

Other than Facebook, Alphabet (GOOGL) - Get Report , Apple (AAPL) - Get Report and Amazon (AMZN) - Get Report are also under scrutiny by U.S. antitrust watchdogs, with the DOJ, FTC, state attorneys general and Congress involved in the inquiries. In a June speech, Asst. Attorney General Makan Delrahim suggested that regulators could take an expansive view of antitrust rules as they relate to digital services, describing tech acquisitions as potentially problematic "if the purpose and effect of an acquisition is to block potential competitors, protect a monopoly, or otherwise harm competition by reducing consumer choice, increasing prices, diminishing or slowing innovation, or reducing quality."

While there's nothing necessarily illegal or improper about buying smaller competitors -- or competing fiercely on products for that matter -- regulators may consider whether the main intent of an acquisition or other conduct was to quash competition, rather than improving a company's own services.

Signs that a company has constructed "artificial entry barriers" to competition is a potential red flag in the eyes of antitrust regulators, according to Kevin Arquit, partner at Kasowitz Benson Torres who previously led the FTC's Bureau of Competition. One famous instance of this type of anticompetitive conduct was when Microsoft (MSFT) - Get Report restricted the ability to uninstall Internet Explorer in favor of Netscape, which was at the center of a successful antitrust case against Microsoft 20 years ago. 

"They look to see what nascent competing technologies there are out there, and they look at the dominant firm's conduct and ask: Is the practice that's being challenged a practice the company has any reason to engage in, other than to keep out competition?" he told TheStreet.

Facebook, Apple, Alphabet, Amazon and Microsoft are holdings in Jim Cramer's Action Alerts Plus portfolio.