Ask for their email, then pen and paper. Or keyboard and a local, maybe synchronized contacts list.

No, not sarcasm, no, not joking. I had decades old email addresses of friends I pinged when the lockdown started, and the vast majority of them got to the intended person.

EDIT: yes, one by one. If they matter to you, they should worth the time.

What's sad / frustrating / predictable is that they had an open API until early 2016 to export contacts. No mistake they shut that down to lock folks to the platform I'm sure...

I could do that personally, but I don't think telling other people to do it would very effective.