you're studying to be a better employee to advance your career? or is just for a title (ceo or phd)? the startup gives you a shortcut to advance your career. you dont mention your age, if you're young, you can usually get away with making a career switch or two. the guy that goes for a startup wants to (probably) make a huge impact on the world in a short time. the guy that stays in big company will have more specialised impact but it probably wont be a headline maker of an impact.
we cant decide for you. but if you think of the two choices as those two extremes (big impact vs specialised impact) then maybe its easier for you to chose. good luck bossman
Working for Google/FB shouldn't be actual goals. The actual goal probably should be to do something interesting and making it big. So, even if you were to do the PhD, you might still be at crossroads figuring out what to do next. The startup, if whatever you said works, should give you enough to explore and understand your boundaries and in that process if you feel that you lack something that can only be fulfilled by a PhD, you can always do it later. You may not get such an opportunity again soon.