Wow, I thought that the professional growth potential from a career standpoint should be the biggest factor to consider, but your answer has left me wondering whether that's really a thing..
FB has some problems that very few companies in the world have... but they've solved most of those problems at this point and given the size of the company and engineering talent it's hard to imagine assignment of some of those problems being given to someone with little time spent at the company. I.E. you'd probably have to wait till you're pretty deep in at FB to be doing the work that you would want to be doing.
How many cool problems could you dig up and solve in that amount of time though? You run the show in your current situation... there's no reason you can't grow the scope of your business to incorporate the professional growth you are looking for. My $0.02 at least