I think there is a reasonably healthy Brockley one. The Sydenham forum also seems quite popular. I suspect that the 'East Dulwich Forum' is busier than this one, and there is also a longstanding 'Virtual Norwood'.

It's hard to judge which is busiest, most popular or best but I think the other 'SE' forums to which you refer are a bit of brand expansion by our webmaster, no? They are a bit more recent so they may take a bit more time to get off the ground, as this one has.