That is a bad analogy. You can start a family diner tomorrow and compete with McDonalds. There is nothing preventing that 80% to eat at your place.
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.
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.