You are right. Nothing better than a company using your private messages for targeted ads!
I mean, what is the alternative? You can pay subscription fees(noone would), or see generic ads(no company would advertise due to next to zero CR).. Either way, facebook is dead.
What makes companies like Facebook, Google etc unique is that, their ability to appease to both sides of the market.
If they don't, they simply cannot exist. And lack of privacy is a price we have to pay if we want these services in our lives.
Maybe I'm morally corrupt. Idk.
I'd be happy to pay for the marginal cost of a quality, highly scaled, private messaging service for me and for everyone I communicate with.