Honestly, from launch, I could guarantee that the population has declined, and I honestly from the stuff they -do- do in the game (factions as friction, combining servers, et cetera) all point toward the devs working with a limited population. I mean, RIFT is a very niche MMO to many.
At the same time though, I kinda have the mindset of "Saying the game is dying is irrelevant as long as you have your group of friends and people to play with, and the content keeps flowing."
I still get a dungeon queue within a minute or two as a healer, (I know almost instant for a tank :P). I can find a raid group PuG pretty much any time during the day. I still have 10-12 guildies around most of the time to chat with, the game has a huge expansion on the way as well as updates every 5-6 weeks. I feel content with the game.