Network effect let Facebook attract billions of users who joined to talk to people who were already there. Switching costs turned those users into hostages - blocking interoperability means users have to choose between friends and fleeing the platform.
https://www.eff.org/deeplinks/2021/08/facebooks-secret-war-switching-costs

Facebook’s Secret War on Switching Costs
Update, October 1, 2021: The original version of this essay incorrectly stated that Metcalfe's Law dictated that the number of connections in a network doubled with each new user; that has been corrected, below.When the FTC filed its amended antitrust complaint against Facebook in mid-August, we...