The population is excellent now, and because it is a more worldly game now (all Euro and Japanese servers are here), the population is much more constant. It used to be population peaked in US primetime, and was pretty low other times. Now, population peaks at about 6 PM EST. the populations usually exceeds 2000 though from noon to about midnight EST.
Zergs are definitely the main thing, but a stealther should know how to avoid them. There is labyrinth where solos go. It is more of a dueling atmosphere there, but if you want 1 v 1's, that is a solid place to go.
the main portal keeps are always busy, but fights usually get jammed there. Populations is fine though, it is just what you make of your time. If you want to find solo fights, there are plenty of people out there that say fights are easy to find. Stealthers have been given so much love lately, I'd say they are on top of the food chain now considering they get to pick their fights, and can go toe to toe versus a heavy tank now with all the shield nerfs and duel wield love.