Rob Fahey has an editorial piece up on GamesIndustry.biz talking about ways to make MMOs massive. The article states that the question devs should be asking is not "How do we beat World of Warcraft?"; it's "What can we learn from World of Warcraft and how can we co-exist with it?"
He defines two ends of the gaming spectrum. The most accessible games are found on services like Facebook, while the least accessible example would be World of Warcraft. This may sound a bit strange since WoW is probably the most accessible of the subscription-based MMOs, but he's using it due to its extreme popularity given the fact that it still has fairly inhibiting install, patching, and strategic components.
Rob believes games that fall in the middle of this spectrum are likely to be where we'll see the largest growth potential. He would consider Free Realms to fit this middle-spectrum description. FR has a middle-ground business model, is fairly easily accessible, and marketed at the youth and female demographics. Basically, Rob believes MMOs will become more massive if they can accomplish what FR is currently aiming for and achieving.