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..

I think it is legal but it risks anti-trust scrutiny.

Google pays Apple tens of billions to be the default iPhone search engine - this seems quite similar.

No it’s definitely not illegal. Companies hire consultants, influencers, contractors all the time to help them improve their product, brand, positioning etc. It could be an issue if the influencers have a contract with TikTok and Meta encourages them to break it. Then it becomes tortious interference. Also, IANAL but I took a business ethics class in college that covered this topic.

Contracts are a fundamental part of common law, why would it be illegal to enter one?

Also, it’s a big leap to prove that exclusive agreements to post on a specific platform is attempting to bury a competitor, TikTok (and every other company) is free to offer exclusivity contracts as well.

How is this any different than the a sports league giving exclusive rights to their product to a specific network...?

If you phrase it like that, sure. But I'm positive that's not how meta phrased it.

Everyone at Meta needs to take a "communicating with care" course every year which exactly trains you to hide your true intentions and not leave a paper trail of anti-competitive statements.