Facebook Inc. for most of the past decade was Silicon Valley’s 800-pound gorilla, squashing rivals, co-opting their best ideas or buying them outright as it cemented its dominance of social media.

Now the knives are coming out. A number of Facebook’s current and former competitors are talking about the company’s hardball tactics to investigators from the Federal Trade Commission, as part of its broader antitrust investigation into the social-media giant’s business practices, according to people familiar with the matter.

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