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.