I'm not sure how useful this is [for me]. All the computers I access Facebook from I already have a synced dropbox set of directories so choosing one of those is just as much effort as choosing the from the Dropbox picker menu.

I can see that it could be useful for people accessing facebook from public computers or other temporary devices.

When I first read it I assumed you got a directory in Dropbox that when you posted a document to it, it added it automatically to the linked facebook group.