They /could/ do it quite easily. I can email people who have different kinds of email account with no problem. The Jabber / XMPP protocol was designed to link messaging systems in this way.

While it would be great if they moved to a pure Jabber federation model or similar, I very much doubt they will do that, however, unless someone like the EU can get their act together and force them to do it.

I think they would link all your accounts by phone number. This may not appear directly in the front end app.

Most likely though in the background they will use these links as ways to package your identity up when selling to advertisers. Thats what I'm guessing is the end goal , enrich their stagnant platform (FB) with their growth platforms (WhatsApp and Instagram)