The big difference is that Blizzard's users are also customers, and their Real ID dick move would end up costing them real money; having a huge backlash would probably translate in thousands of cancelled WoW subscriptions. They weren't going to lose revenue on that, so they backtracked.
Facebook, however, doesn't get its revenue from their users, but from advertisers, and those are the ones that push for real names. The only way Facebook would backtrack on this stupid policy would be for users to deactivate their FB profiles en masse, and in sufficient numbers for advertisers to notice and take action. But I doubt that will ever happen.