Sounds anti-competitive in the UK. Using dominant market power to censor.
Facebook claims to be a social network. Those types of networks normally DO NOT allow you to promote other social networks.
Even review platforms which are a bit more communication in nature block posting other sites reviews.
I don't see how that makes it legal.
Herey other examples of potential actions that would also clearly qualify as illegal anti-competive behavior:
Microsoft could decide to block the download pages for Chrome and Firefoz from being shown in IE.
Google could block results related to Bing in their search engine or browser.
I don't see how this behavior by Facebook is any different. If it can be shown that this was done deliberately by Facebook, I have little doubt that it would also qualify as anti-competive behavior.
When you first search on bing for chrome downloads they will put up a big edge promo above the result. After you download they will pop up a box asking if you are sure you want to switch as edge is "Faster and more secure".
The problem for mastadon is it has a TON of content that is against facebook policies. So facebook can simply say - users are reporting this crap as spam - we've blocked it. Done.
European law tends to favor maximizing competitors. American law tends to favor maximizing consumer value. At first glance, these can be considered equivalent, but they differ at the margins (which is why, for example, Amazon keeps getting hit with antitrust in France but not in the US).