When real monopolies exist and are dangerous, we need to be able to tell them apart.
Facebook is still far from the textbook definition of monopoly, but this anti-competitive behavior is not good.
acting with the knowledge that they can stem the growth of a competitor and flipping the switch
Although if it is or not a monopoly and an abuse of market power is dubious, but it clearly is anti-competitive.
This is similar to Amazon shutting off Google Products from any retailers using its platform. It is anti-competitive, but the jury is out on whether it is abusing market power.
TL;DR: you can try to shut off your competitors from the market, as long as you're not abusing your market power to do so.
PS: The Hacker News definition of monopoly has nothing to do with the economics, policy and antitrust definition of monopoly.
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).
Removing the access of a competitor in retaliation for them competing with you, when you give it to other players is anti-competitive by definition.