I do wonder about this problem.
Partly, if people know that you're not on facebook, it might help make them think there's a good reason for that (but don't become that person who always rants about how terrible they are at any opportunity, or no one will listen to you).
Perhaps just drop little stories into conversation about what a scummy company they are, eg "you know that they won't pull a political advert even if it's a bald-faced lie?", "did you hear about so-and-so, getting scammed on facebook?".
Might also be worth pointing out how much of the content is just designed to make you angry, so you'll click on things "I just found that I was much happier after I stopped using it, my blood pressure came right down".
Also there's pointing out the lies, "did you see that thing on facebook? it said X, but it was complete bollocks, and it's just set up by Y in order to sell you Z".
Some of those might work.