Here's something that has been in the back of my mind for some time. It's not 100% reliable, nor is it adequately tested but it's a thought:
When I start a conversation, when I am the initiator in the exchange, I use my favourite mean of communication (email or whatever). When I receive a message I go along the sender's favourite mean of communication.
It depends on the practical need for synchronicity. When async is ok (intervals of hours or days betweeen responses are acceptable), even a small group conversation can be migrated from FB to email, with a short explanation.
It is also possible to receive a message on one channel, while replying on another channel.
Well, my preferred form of communication is e-mail and thus I tend to start conversations via e-mail (if an instant response isn't required). Guess what? Some people started replying to my e-mails via Facebook! I am somewhat okay with using FB as "the new sms" but I don't want to store my personal correspondence on their servers.
When I start a conversation, when I am the initiator in the exchange, I use my favourite mean of communication (email or whatever). When I receive a message I go along the sender's favourite mean of communication.