This game goes through periods of extreme lull and boom from what I can tell.

Beginning of any major patch (2.1, 2.2, etc.) major activity, people rushing to do everything, party finders everywhere.
Middle of any patch, still some activity, things have largely died down at this point
Near end of any patch (End of cycle) almost total ghost town, people largely logging in to do dailies/hit caps/do Coil and then log out again. Doom threads start appearing on the forums.