I'm one of those people. I'm only using messenger, but since I message people from desktop (and I don't want to install their client, they know enough about me already), I still have to log in to the base Facebook page. After a while I started using the Feed Eradicator plugin (or what), but unfortunately I still have to log in if I want to be able to message my acquaintances.
Why am I still using Facebook messenger? Back when most of the users were still unaware about their practices, all of my friends and family joined and now it is their main way of communication. There's no way everyone important to me would just start using a second social media platform for me. Also, I can't make calls from desktop WhatsApp (which could still have a chance to become a substitute for most of the people I know). Anything else is just too niche for most of them to install for the sake of one person. Or three.
I wish there was a good practical solution because I'd prefer not being part of this ugly machine, but until then, I'll have to stick with it.