From a technical point of view, Facebook building a token will be trivial. There's more tech and AI in their messenger bot platform, than in a simple, centralized token, so throwing in a "coin" is really, really simple for them.
The only question stopping them to do it is: "how much more money a token will bring us, than we're bringing in now, with our current business model?"
I'm referencing issues regarding regulation - Facebook has already been approved (along with Google, if I remember correctly) to operate within the technical framework of a bank in the UK, I believe. Also M (the AI layer inside of Messenger is/has been canceled for the meantime).
They don't need to launch a token to get a cut of tokenized transactions, they already have the infrastructure to add the layer but perhaps would be savvy to shy away from the politics of initiating their own.