Neither Meta nor Onlyfans are government organizations, they are privately owned and as such can make whatever deals they like. Calling a deal a "bribe" is just silly.
Not really.
1) Meta is publicly traded, and has a duty to shareholders.
2) Antitrust laws limit what can Meta can do.
3) Paying individual executives to take action against Meta's interest is a crime.
4) A deal that involves Meta breaching contractual obligations with other parties or otherwise committing torts (e.g. libel) is also actionable.
There are more exceptions, but that should be enough.