Why the down votes here? It is pretty damn amazing that not only has Zuckerberg maintained control but he has also been smart enough to bring in talent to help him, personally, scale. It would have been easy for Mark to simply say, "Eh, I need to hire a CEO", or for him to listen to the countless investors and advisors that are almost certainly hinting to him that he should do so.
Why did Ev step down? No one really knows. It, almost certainly, is not because doing so will enable him to now focus on product. As CEO it is quite easy for him to delegate nearly all of those responsibilities he wishes to ignore to Dick. No, giving Dick the title of CEO almost undoubtably has beneath it many more subtle reasons, some of which might simply be that Ev convinced himself he needed to do this for personal reasons. We'll never know, but IMO the "so I can focus on XXX" reason is never the real reason.