herc, Murray's World also has over 4000 members; however, only 15 to 20 posters post on a daily basis but since they post almost all day, it keeps the forum very active.
Tennis Frontier has about 200 members but the active posters make it a very nice place by posting some quality posts. Not as much chit chat or going after each other for no reason and more to the point. I'd rather have the onset of TF.
I can't honestly say that T4U was ever all that good anyway. Back in 2008, 2009 and 2010 I had seen a lot of fruitless arguments going back and forth by Federer and Nadal fans. That place has always been about that. Same goes for Tennis Warehouse. There are just one too many threads/posts about Federer and Nadal. Very off putting for me.