I'm wondering if the OP is asking because he hasn't actually got GT5 yet?
It's pretty easy to find out how active online racing is if you have the game...
Another thing the OP doesn't spell out is his experience level... To a certain extent, with GT5 having been out over two years, and other racing and crashing games have come and gone, if the OP is asking simply whether he can find a room to drive around and crash a lot in, there may be other games where he can find more players like that. The 'crash and burn' drivers tend to move on with each new game, especially as many of them specialize in being 'crash and burn' type games (which GT5 really isn't, with way underplayed crash effects and sounds).
But if he's looking for a dedicated player base intent on sim-ing proper racing, outside of iRacing he's hard pressed to find a better race community than the GT5 online one.
No it's not. In the past few weeks, GT5 has drastically died down. Go to the event section and you'll see. Most of the events on the front page are events that ended weeks ago. GT5 had a great run though, no doubt!
eciracing, I think the drop off is also related to burn-out.
TBH, the pick up races are still doing well, which may be due to the winter times and lack of daylights.
With the league races, such as the one you organised, I really want to do well and get more points, thus I put in a lot more hours in practices. Together with the 24 hours of LeMans A-Spec, I was totally burned out. I was racing almost 4 days a week (plus 3 days practices in between), that really was too much of my evening hours gone (but heck, that was fun!)....So I cut down my racing days to 2 (may be 3 times a week max). I generally consider that the Monday and Saturday PSN Community races are pick-up races, therefore little or no practices are required.