It's going to be a combination of asking people to get in touch with you over another service and letting go of some people who aren't willing to do that. This would also mean getting fewer updates, but depending on you, your age and circumstances, it may not be a big deal not getting to know certain things. You could also ask a few trusted people who continue to stay in touch to pass on anything important from Facebook and its services.
I'd recommend Telegram because of its superior UX, though normal chats are not end-to-end encrypted. I wouldn't recommend Signal for anything other than voice calls (mainly because it purposely prevents backing up information, thus preventing moving conversations from one device to another). If you prefer a free and decentralized option, Matrix (matrix.org), with the Riot (riot.im) client on multiple platforms, is something to try.
His apparently doesn't. Same for mine: If you asked me whom of my friends and family use Facebook, I could not name anyone. Only indirectly: Whatsapp is pretty dominant.
I'd recommend Telegram because of its superior UX, though normal chats are not end-to-end encrypted. I wouldn't recommend Signal for anything other than voice calls (mainly because it purposely prevents backing up information, thus preventing moving conversations from one device to another). If you prefer a free and decentralized option, Matrix (matrix.org), with the Riot (riot.im) client on multiple platforms, is something to try.