It is retaliation for another business competing with you. That is literally using your market power to try to reduce competition.

But taking anti-competitive actions by itself isn't necessarily an issue, but it is a problem if you're deemed to have significant market power in a defined market.

Essentially: if Facebook is deemed to have significant market power, this could be a problem, if it doesn't have, then it isn't a problem.

Anti-trust relies heavily on the precise definition of a market (which is a subject that Hacker News comments generally ignore when crying wolf, I mean, monopoly).