solsociety is a bunch of fucking ricers, but at least its active.
Automotive Tech ( www.automotivetech.org/at ) has a small sol forum that is great but VERY slow. Im a regular over there and on their Off Topic board.
dsoc ( http://www.delsolownersclub.com/ ) is the new kid on the block... not too busy yet but maybe soon.
then there are regional sites, too. closest is Team Sol Florida ( www.teamsolfl.com )