> These are people I care about, who care about me, and who are, today, using FB almost to the exclusion of anything else because they find it convenient and have given up nearly everything else.

Just to be clear-- they send and receive rich content over the FB app (taking and forwarding pictures and/or video, etc.), but they don't know how to send/receive that content through text messages?

Not trying to be unsympathetic-- it's just that every non-technical user of a smartphone I've ever seen degrades to text messages.