The Facebook groups that could be considered competition to
Cyburbia aren't so much discussion groups, as they are news and meme aggregators. They're a lot like early Reddit -- people post links to some article elsewhere, and comment on it. However, some Facebook groups are VERY busy, and
have a lot of media buzz.
Cyburbia's real competition includes blogs, a planning news aggregator site, and Reddit. Among urban planners, blogs serve as a kind of scattered-site message board -- people post messages on their own sites, rather than a subforum on a common message board. Reddit's urban planning subreddit is a bit of an echo chamber (most posts are about urban living, "suburbs suck", transportation/transit, and projects in superstar cities), but its mindshare among younger Millennials really hurt us.
Old-fashioned email lists are an ingrained part of urban planner culture that we've had to live with since starting the message board in 1996 (!). Many planners have told me they prefer listservs to message boards because they're "more serious" and "more work-related". If a supervisor is looking over your shoulder, it's better to have Outlook open than Firefox or Chrome.
Anyhow, back to the OP - I'm working on posting the Cyburbia RSS feed to the Cyburbia Facebook group.