Private companies are regulated all the time. Look up the rules for any utility or communications company. They banned politicians, so that's going to get laws passed.
Social network and communications have nothing to do with each-other? are you reading what you are writing? How is Facebook messenger call different from a phonecall of the 21st century?
It's impossible to use an alternative messenger to communicate with people who use FB messenger. Unlike e-mail providers or phone operators, these newer web services don't have any interoperability, they're closed systems.
To switch from Facebook, a person gonna need to convince all people they communicate with to switch as well - to the same alternative messenger. That's very high cost to switch, borderline impossible to do for many people.
I have at least 7 apps on my phone now that I can use to call someone over an app.