https://ubarchive.net/ has a list of some places people have scattered to, including
NYCityMoms.com, you are here (lookin' good!)
urbanbaby.net a possible UB clone under construction by a UBer (nothing up yet)
Reddit.com/r/urbanbaby/ is live again as of yesterday, but not very active.
This is the most active right now, except vs. the YBM main board, which is its own thing.