I'm happy to leave the FB link up till the date of the meet up has passed. We have a rule re FB links and usually remove them but in certain circumstances I'm happy to leave them to stand. This is one of those instances :)
FWIW we do have a group thing on the forum for members to set up groups (open or closed) but realise that its not got quite the same appeal as FB.
I do try to keep in mind also not everyone is on FB or wants to lose their anonimity to the FB world in the jump from pretty anonymous to the masses on a forum to FB where everyone can see your name, profile pic and whatever else you choose to not keep private.