Most of Facebook's users are international. There is a chance that what is "liberal" in America is simply something that runs counter to the American conservatives disciplined beliefs.

Example #1: When a prominent American conservative does something like shutdown the government instead of compromising (remember compromising gets you booted at the next elections see: Paul Ryan). To most of the world (liberal and conservative), this is nuts. Facebook bubbles news reports about how "crazy" this is to the top. American news media will most likely call this a liberal bias when really it's just an international one.

Example #2: If facebook, editorializes a giant Ice-sheet melting and how that is related to anthropogenic global warming (something most International conservatives have accepted), most American conservatives will call that to be liberal bias. Note that this is not about policy decisions, most international conservatives don't agree with Carbon Taxes for instance, but one of just straight facts and assumptions.