[ru552]

Of course it's legal. They offer you money to exclusively post on their platform. Joe Rogan got paid to exclusively podcast on spotify. All of the talking heads on TV news are paid to exclusively be on their specific channel (ABC,NBC,FOX,etc.) The twitch streamer, Ninja, that got paid to move from twitch to Microsft's failed Mixer platform, etc. etc..

[singleshot_]

This is an extremely low-quality answer which in no way correctly represents the complexity underlying the question.

GP would need to talk to an antitrust expert to learn more useful information concerning the distinction between being a monopoly, and illegally leveraging monopoly power. I am not such a person, so I’m not going to say more here.