It doubled from 200 to 400 in one week, why couldn't it do the same once it reaches 500 here shortly? Think logarithmic.
Definitively possible, but logarithmic (exponential) growth cannot last forever in real world. It has to slow down and stop at some point.
The growth has been faster than exponential, though. If you look at 1-year charts, the growth appears to be exponential in the interval between Dec 2012 and Nov 2013. Then something happened in Nov 2013.