It has a monopoly on certain forms of connection and communication. You could easily say the same thing about telephone companies, that it was "absurd" that they had a monopoly on communication - after all, people communicate in many other ways beyond the telephone: mail, face to face, telegram, and so forth. But we still treated the telephone companies as having a monopoly on a critical form of communication. And we took the appropriate steps: broke them up or treated them as utilities.

Facebook has over 3 billion users. Nearly 40% of Earth's population. Many of whom communicate more actively through it than they do with telephones, mail, or any of the other forms of communication that we treat as public utilities. It absolutely fits the bill.