I disagree-- they both have images to maintain-- him being happy in a relationship and her getting some of his shine (and hopefully landing some more film roles).
Cheating rumors have been around him for a while now. Plus Olivia Munn (or her people) basically gave US Weekly the details of him hitting on her. Interesting they waited until after promo from The Social Network was over before they announced the split. I bet they have not been together for months but just waited to officially announce it now.