I started playing just over a week ago, got the expansion, but during the normal leveling to 80 part I still saw plenty of people in the normal zones. ArenaNet has done a good job of making sure that those zones are useful, most notably for map completion and because in the expansion there are mastery points that can only be earned by playing in the normal part of Tyria. So there will be people in those zones, even just to play content related to the expansion.
With that said, going from 1-80 this past week I had no problems finding people to do random events with. As much as the expansion is providing content for veterans, it has also generated a lot of hype that brings in new players such as myself.