@gentlemand, that $500 million spike looks really weird (around 1/1/18)... Must have been when a lot of people pulled out as their money hasn't returned to this market yet (since we aren't at $20k).
It's more likely it was caused by the very high fees at that time, 800 sat/byte was "normal", resulting in $25 or more in fees for a small transaction, and many thousands of dollars for a transaction with a lot of inputs and outputs.