One way monopolies form is through economic efficiency. One major cause of that efficiency is positive network effects. Network effects are the economic effects multiple people gain when they use the same product as one another; this is particularly obvious in social networks, which get to be more fun when your friends use them, or when cool, smart people use them to create guides, stories, videos, music, etc. that you can enjoy. Social media tends to suck when there aren't many people on it, since nobody's really talking about anything you want to talk about, and if you post a lot, you feel like you're shouting into the wind.

However, competition and variety are still good things. They still help advance technology, and help keep firms honest. Monopolies take advantage of their consumers, because they can. Because they have no competition.

Is there a way we can have competition and variety while still taking advantage of positive network effects?

Yeah, federation. Extend one network across any number of services that want to participate in the network. The network can grow arbitrarily big, while the market remains competitive.