I had a group that inadvertently tried it. We started with custom forum software that was paid for and being actively updated by the site owner. We used it for more than a year, but it was clearly inferior to Facebook's platform that when someone created a private group for us, the forum basically died within two or three weeks.

I think the owner struggled with the idea that we wanted all (or most) of the features of Facebook. Even such simple things many people overlook like the information density on the page.

Wade.