-Talbot-
The reason why it wouldn't work is that it's incredibly impractical.
Think of it this way, three mates are going out on the pull. They can either:
a) Go to a gay friendly bar
or
b) Log into Facebook. Find a group and suggest to that group that everyone meet up at a certain pub/bar/club on Saturday night.
With a) the three friends can go to the bar, stay there, or head of to another, or any other number of gay friendly establishments in the area.
With b) they have a number of like-minded individuals in one bar. They're stuck there.
Also, there will never be a FB group which has numbers like those in bars in metropolitan areas. It might be in option if you live out of the way, but if you're in London, Brighton, Blackpool, Manchester, Birmingham etc it makes no sense to ever go for option b) whilst option a) exists.