I very much doubt a subscription based messaging app would take a big enough share to affect Facebook.
Not to say that business can't successful make money. There must be people out there willing to pay for privacy.
there are plenty trying.
I think most would be targeted at business professionals that need a certain level of privacy. I don't see how a service like that could win over enough domestic users to make a dent in the market while whilst the are so many free alternatives with great features