Network effect at work
The sort of thing Facebook is ends up as a monopoly because of the network effect where, in essence, users attract each other and where a competing upstart wouldn't get anywhere as the people you want to aren't there; as a natural monopoly of a sort it ought to be run as a public utility.
An ideal approach could be splitting Facebook itself into two parts:
- a regulated utility providing the technical core of the service on a common carrier basis, and
- an user facing part operating under competition
i.e. the former would used trough a well-documented API by the latter which would be just one of competing implementations paying fees to the regulated part. In practice, I suppose, regulating the existing monolith will have to do. Also, there is obvious opportunity to improve competition by cleaving off Instagram and WhatsApp.