Tell them no one expects them to delete their Facebook account right away. Post here what you would have posted for them on Facebook, even if none of your friends are here, yet. This way they wouldn't see an empty profile here, once they join friendica.
You could also write them a small text on how to join friendica and things work here, to make it as easy as possible for them.

A killer feature would be, like someone here (I guess) already mentioned, to be able to import a Facebook export. I just initiated an export of my Facebook data. I don't know what it contains and if this could work. But that would be awesome. I guess many people don't like the idea to lose their complete history on Facebook. But someone would have to implement such a tool or functionality.