Price of acquisition is not relevant to anti-trust. It is not like a monopoly is OK as long as you paid enough for a monopoly.
Facebook's business is ads.
Google + Facebook have a virtual duopoly on online ad revenue.
(Still does not mean this is not a problem)
Monopoly is defined by market share in a specific regions, not by the unavailability of alternatives (1). So it doesn't matter if there are "many other messaging apps" what matters is what's being used.
(1) https://www.ftc.gov/tips-advice/competition-guidance/guide-a...
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.
Whenever there is a network effect, anyone with a monopoly on the network has a monopoly on a market.