80% of the country using facebook 'regularly' is not a monopoly. And even if it were, what are they monopolizing, social media in general? Just because 80% of the country visits McDonald's regularly does not make them a monopoly.
However, you cannot start a social network tomorrow and expect to make a reasonable income. People will not join your network, because their friends are not there. Facebook holds a significant part of the world population hostage, because they cannot 'dine' somewhere else. Contrast this with a situation where social networks are federated. You could opt to go to a competing network because their service is better, without losing contacts to your friends.
I would not know if that qualifies as a monopoly legally, but there is clearly a huge difference between McDonalds and Facebook.
They tried to buy Snapchat too.
Some interesting reading on how we might measure the unusual cost of Facebook's monopoly to society.
https://stratechery.com/2017/facebook-and-the-cost-of-monopo...