I think most people will just suck it up and validate their IDs.
These sites have such a huge reach, you'd pretty much be starting again if you went to anywhere else. No-one's going to want to do that, so they've got everyone where they want them, for sure. Facebook is used by businesses too. It's often easier to find information on there than on their websites.
I think you'll be able to use YouTube if you're not logged in, it will be limited, though.
There are other options for messaging - signal, telegram, but I think they'll be covered by the "ban".
I've heard WhatsApp will probably be included as well. Something I've only used once before anyway.
I just had a chat with Sharn about this - our non-negotiables are probably:
We can live without the others. The thing is, we're not children, and our accounts on some of these platforms are >16 years old anyway!
Oh, wow! I'm pretty sure my accounts aren't that old, so would require further proof of age. It will be interesting to know if they still try to require proof of age from you, though.