Well, if you compare the population now to the population during the first few weeks or months of the launch, then yes, it looks like it's dying, according to people who played since beta or first few days of the game release.
I joined last December, when some people (including AngryJoe) already left GW2, and I think the population stays the same. I see people left the game for good, and I also saw some new players every month.
The stacked server will always have the most population.
Blackgate seems a good choice if you want a server with big population (PvE and WvW).
Other than that, you can try Jade Quarry. I heard from my brother that the WvW community there is nice, and they have big population, too.
For MM build, I saw one necro used it in sPvP, but it was a zerker/glass MM necro, not a bunker one, so yea the necro was quite deadly.
In PvE, I don't know. I don't do PvE/dungeon much coz I bought GW2 solely for playing WvW xD
I still play every day since I'm still addicted to WvW. ^^;;;
My IGNs: Euphemia Hime, Chibi Asura San, Vanille Morgana, Pinky Pearl, Avicenia