Answer my question first.
If Facebook wants to be an arm of Obama, then it's their sword to fall on but they also held a town hall with Bush after he left office. An argument could be made that since Obama has used it as a platform (government policy) then it could potentially be up for potential legal action if it limited the free exercise of an opposite political view point but i'm not sure how that would turn out.
From a purely laymen's point of view, if Freerepublic let Palin hold a virtual town hall here, I have no qualms of not allowing his opponent speak here.
The size of the website shouldn't determine. Too big to run their site as they please. I would think it sucks but if you disallowed the other half from their point of view, you will find FB to lose a crap load of page views (which is a ton of money in ad sales). They don't want that.