Facebook has 1.86 billion monthly active users as of Q4 2016[1]. At that time approximately 3.4 billion people used the internet. So a little over 1/2 the internet-using population, and 1/4 of everyone on Earth, has a Facebook account that they access at least once a month. If you drill down a bit about 80% of American adults use Facebook regularly[2]. You may know tonnes of people who don't use Facebook, but I think that's actually quite unusual. Outside of our HN-reading, privacy-interested, technically-competent bubble the overwhelming majority of people use Facebook, and Facebook does have an effective monopoly.
[1] http://money.cnn.com/2017/02/01/technology/facebook-earnings... [2] http://www.pewinternet.org/2016/11/11/social-media-update-20...