The Federal Trade Commission is examining Facebook Inc. ’s acquisitions as part of its antitrust investigation into the social-media giant, seeking to determine if they were part of a campaign to snap up potential rivals to head off competitive threats, according to people familiar with the matter.

The company’s acquisition practices are a central component of the FTC probe, the people said. Facebook disclosed the FTC’s investigation in its earnings announcement last week but provided few details.

FTC investigators are examining whether the company and its CEO, Mark Zuckerberg, purchased technology startups to keep them from challenging Facebook’s empire, the people said, some of whom added that the FTC has begun reaching out to the founders of such companies.

The tech giant has acquired about 90 companies over roughly the last 15 years, according to data compiled by S&P Global. Among those companies are the photo-sharing app Instagram and the messaging service WhatsApp, which bolstered Facebook as a dominant force in social media and messaging.

Facebook didn’t immediately respond to a request for comment. The company said last week that the FTC was investigating “in the areas of social networking or social media services, digital advertising, and/or mobile or online applications.”