Monopoly definition: the exclusive possession or control of the supply of or trade in a commodity or service
Why doesn't FB let other businesses else access friend lists via API? or send people messages via API? Same with Google and all their data about you. They just can't do it.
They love to take the most bad-faith reading of the API docs to say that Facebook is giving your data away (see their story about netflix/messenger, or Microsoft's Facebook client for Windows phones)
FWIW, Apple is a monopoly as far as iOS app distribution goes. They act as if they own the relationships between app developers and their users. This stifles innovation. They also keep playing moral police even though they have no obligation to do this (as in, "you have to change the ToS of your service to be allowed to have an iOS app").
It also does its best to control the experience end-to-end to make sure no one can innovate their way out of these algorithmic feeds and ads. People aren't building better client apps because they fear getting sued. Imagine using Facebook but doing so via a comfortable UI that puts you first, gives you complete control, and allows you stuff like cross-posting to other services. This is their nightmare.
And another problem: the US government is on their side from what I see happening (disclaimer: I'm not from US). It should be such that if they do try to sue you over a browser extension or an unofficial app or whatever, they would get laughed out of the court because you were in your right to do what you did. But unfortunately, there are apparently laws that do allow them to sue people trying to improve their own user experience on someone else's online service.