If the difference is real (I've not seen it) then perhaps it's a result of more users becoming aware of the option to disable "Pair my social actions with adverts" via settings: http://www.facebook.com/settings?tab=ads&section=social&...

In my own case I've come to see "likes" as a liability and removed all existing "likes" for commercial organizations or any uncommercial org for which I see these paid-for posts. Hardly the desirable outcome for Facebook or the organizations! I hope my reaction becomes more prevalent.