Planetizen is more of a planning news blogging site. It's a couple of years younger than Cyburbia, and it's the founder's full-time job. (Mine, meanwhile, is looking for work, writing a constitution and bylaws for a future non-profit Cyburbia organization, and struggling to keep my head up.)
There's the Reddit /r/urbanplanning subforum, but it's focused more on trendier Millennial- and Generation Z-centric urban issues; mainly, sprawl, transit, and housing in large HCoL cities. Some at /r/urbanplanning think we're a bunch of disgruntled old planners, others think it's a good resource for career advice and more nitty-gritty everyday issues. You're not going to find a discussion on the rationale for subdivision performance guarantees or rooftop mechanical equipment screening on /r/urbanplanning.
There's also general urbanism/skyline forums; SkyscraperCity, SkyscraperPage, and Urban Planet. They're primarily city and tall building fan sites, with a focus on individual development projects, and only a little bit of planning discussion. They inspired a bunch of more localized spinoffs, like Urban Ohio, NYC Nimby, and Urban Toronto.
In the past, when Cyburbia has been off the air, I've made temporary forums. Depression kept me from doing that this time. It's also why I'd like to find a way for Cyburbia to be self-sustaining, without a single point of failure.