Which platform are you thinking about purchasing for? This could matter. I can only speak for Xbox.
On Xbox, there are lots and lots of people still playing. I play every day. There is also more DLC coming.
There has been some diminishing of the population due to the most recent DLC, which caused some lag/frame-rate issues as well as dash-boarding/game crash incidents. PPl tend not to want to deal with these issues while playing. Devs say they are working on resolving the issues. Once fixed, I'm guessing ppl will come back.
All it all, it's still a great game and I'm hooked. It's definitely not dead. Not yet anyway.