I glanced (literally five minutes) at the servo library code and it looks like it uses Timer1 initially. This is a 16 bit counter. It puts it in ordinary count mode and enables overflow interrupts. With the prescaler of 8, it should overflow roughly 30.5 times per second. This would be the update rate to the servo, not 50Hz
I don't have the source to hand to glance at, but I do have a Servo library project and a 'scope.
50Hz, pretty much on the nose.