FOX are loyalty points at ShapeShift.
In the near future, the tokens will be distributed and can move around (existing as ERC20 tokens on the Ethereum network). At that time, by holding FOX tokens Members will get better prices on trades in the ShapeShift Platform. The more FOX tokens possessed by a Member, the more discounted exchange volume that Member can do per month. Stay tuned - more details will be available soon.
Oh...and be sure to sign up so you can begin earning your FOX!