I'm so anti-Facebook now that it's a part of the way I identify myself, and for all that I can't delete it. I maintain contact with a friend in Germany via Whatsapp or Facebook messenger, and in this case it would be possible to use email (which is not nearly as casual as firing off a message in your spare moments) or some other service but it doesn't solve the problem about friend groups.
I have friend groups around the world that my only way to participate in is Facebook. I believe moving abroad is in my future again, and Messenger is detestably the only real way to keep up with my friends back home. Leaving Facebook and Messenger is like leaving a bar I hate; I'm only here for the people and I wish we could go somewhere else.