Monopoly is with respect to a market. If everyone on the East Coast uses AT&T to the exclusion of any other provider, and you could only communicate with AT&T customers by becoming an AT&T customer, that's a monopoly on the customers on the East Coast. Even if everyone on the West Coast used Verizon.
The fact that the Facebook customers aren't all concentrated in a specific geographic area doesn't really change anything. If the only way to communicate with a large population of specific individuals is to use Facebook, they have a monopoly on the market for communication with those individuals.
That wouldn't be true if they were using a federated protocol -- like the telephone network is in real life, but like Facebook is not -- because then you could sign up for some other service and still "call" the people on Facebook.