While I'm thinking of social networking sites, here's a question:

Various friends have contacted me via private message to my Facebook account. For obscure and arcane reasons--possibly having to do with the way I attempted to get access to my Facebook account while in places that block access to Facebook--I have had trouble with Facebook ever since returning.

I attempt to reply to these friends via normal, external email, and it's like dropping a rock into a black hole. Zip, over the event horizon, never to be heard from again.

Is this a thing with heavy Facebook users? Or just those particular friends? There seems to be a population which uses Facebook for everything and no longer looks at their email. Or something.