OFF hours Part II - I just dug up my activity track of August 2012, two years ago. So I was able to directly compare the distribution of activity in LW arena, measured by plane kills&deaths, for the same month in different years.
The result matched and surpassed
my earlier findings In short: US Prime (US evening - Euro late night/early morning) -30% to -35% plane kills&deaths
Euro Prime (US noon) -40% to -50%
Euro noon (US late night/early morning, Australian evening) -50% to -70%
In August 2012 peak plane (k+d) was about 7-8 times higher than during offpeak, in August 2012 it's about 12-13 times as much.
It's obvious that while overall combat activity is shrinking, it's also more and more concentrating on US primetime, possible creating a "runaway" effect.
Note #1: The numbers above can't be converted "to players online" directly. With fewer players online, the kill/death numbers can also sink disproportionally because of the longer transit times, and lower chances to find something to shoot at.
Note #2: I excluded GV (technically GV vs Gv combat only), because pure GV (tank) combat is very much depending on terrains being up during a particular time frame. Thus numbers are jumping more erratically, making comparisons of this kind quite difficult. A map like the recently introduced CraterMa greatly boost tank kill numbers without having significantly more people in tanks at all.
In contrast to this, plane combat data is much less impacted by the maps being up.