Why would users pay when they can send messages sent for free?
A combination of awareness that TANSTAAFL and network effects - RIM sold plenty of BB hardware to people who just wanted access to BBM because that’s where all their friends were, so there’s precedent.
I think it’s pretty safe to assume that even if there were a paid option for FB, Insta, WhatsApp they would still do everything they could to monetise personal data, because that’s just the kind of company FB is, rapacious.