Time - There are amazing perks like really quality free food all the time, a gym on campus with a basketball court and large playing field, shuttles to SF and surrounding areas but I often find my coworkers and I spend less time on that fun stuff that you might expect. There's just so much work opportunity because you are expected to be driving on projects rather than just doing what you are told so you could theoretically work all the time and its up to you to stop that.
Company maturity - Like all fast growing companies, there is a natural curiosity around "how much better would things be for me if I had joined earlier?" because there would have been ever more responsibility and financial potential. Both of those things are still really high but it does raise the natural question of it would make sense to gain the skill sets to understand what makes Facebook work and then consider working somewhere smaller with more financial upside.