fractional reserve can still happen in cryoto. It just means the exchanges don't have enough of what ever currency to cover the mass withdrawal of that currency. They may be running in the red and took some deposits to stAy open with the intention of replacing it from future profit.
This isn't the same as what most mainstream banks are presently practicing, Mt Gox being a good example of what happens when you're unable to generate your own Cypto's!
Interesting article which emphasises the need for exchanges to maintain reserve ratios in excess of 100%, which is light years away from what Central banks allow now.