Hrm . . . That is a good question. In the past, I seem to remember some scripts for IRC that could handle this sort of thing, rolling and then posting the rolls for all to see. I don't remember the specifics (whether it had to be installed by every individual, or attached to the chat room) but if IRC still allows such scripting, that might be a solution and the Facebook page could provide a link to the channel and specific room in question. It's been years, though, since I bothered with IRC, so I don't know if that is a good solution or not.