While network effects exist and are worrying, this is a bit too strong - you can have two accounts concurrently, and many people do. Hence why FB had to buy Instagram and Whatsapp, and why Snapshat has more than a hundred million users.
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...