I'm not aware of a dedicated compact forum. There are however multiple 'big' BMW forums..but not as big as say..bimmerforums.
I think one of the most comprehensive E36 forums is
www.e36.de and this is the subforum for compacts :
http://e36-talk.com/forumdisplay.php?119-E36-Compact Inspite of the fact that the compact has been more succesful in Europe, nowadays there's just a small following for these cars. Most people here are trying to forget the e36 anyway since it's getting pretty old now haha !
The only place I always see other compacts is on the Nordschleife.. usually around 5 different ones every day. Most 323ti's..