I've taken to logging out of facebook on all of my devices. When I really want to check something then I log in, check the thing, and then log out immediately. Oh and I don't know what my password is, it's saved in my password manager so that adds an extra step.

I've also set up facebook notifications to send me emails for calendar invites and private messages because I'd feel like a jerk for missing things that are actually important.

All of this has worked incredibly well. I definitely spend less than 20 minutes on facebook per week.