Here's my go-to argument, lmk if it works for you:

"When you go to the bathroom, do you close the door?

Why? There's nothing illegal about pooping, is there?

Would you be okay with security cameras installed in public bathroom stalls, watching everyone, adult and child alike, who did their business, just to make sure no one was doing drugs in there? (Criminalization of recreational drugs is a whole other issue I could talk about, but you get the point.)

Of course you wouldn't. You're worried about perverts watching the security tapes. By the same token, I don't really want Facebook to know the entire contents of my phone's contact list (no criminals in there -- but what else are you going to do with those numbers?), or exactly what kind of porn I've been searching for (and don't think that just because you're not logged in, Facebook doesn't know it's you! Browser fingerprinting is pretty accurate these days and that's ignoring the fact that your IP address doesn't change). Worse, I don't want every single one of my future employers getting access to my chat logs and asking me about the things I joke about when I think I have privacy."