An updated chart on GV vs plane usage.
This shows true usage, based on time spent in the different modes, for the whole arena.
GV's had been introduced in 2000, before the start of this chart. By 2001, we already had the core setup with a tank, an AA vehicle and a troop carrier. Since then, the GV set has been expanded step by step.
As you can see, the true "GV explosion" happened in AH 1 (AH 2 went live mid 2004), From 2004 onwards, the increase in gametime spent in GV has been relatively small. From 2008 to early 2011 you can even see a sort of decline, which was stopped by the introduction of the improved GV control setup in April 2011.
Again, keep in mind that these are relative values. In absolute hours (which translates into average # of players in GV at a given time), the peak was reached in 2007/2008. On average, we had actually more than twice the number of players in vehicles than we have today.