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.
(From: http://www.nytimes.com/2010/10/03/business/03face.html)
"Ms. Sandberg has focused on building the business, expanding internationally, cultivating relationships with large advertisers and putting her polish on things like communications and public policy. That has freed Mr. Zuckerberg to focus on what he likes best: the Facebook Web site and its platform."