Offering an anecdote with as little commentary as possible, but having gone to a small religious high school, I have friends who argue they'd "never vote for anyone who isn't Christian" but managed to do some mental gymnastics in order to vote for Romney, who during the primaries they seemed to oppose for being Mormon.

Exactly this. If you are already atheist, and people know about it, it is more tenable politically to convert to any religion that exists than to remain atheist. This includes religions that practice human sacrifice. And if you think that is too insane to be true, please remember what country we are talking about, here.

That said, there are some religions that are compatible with atheism, which wouldn't necessarily be recognized by the public as a religion that could have atheists in it, or would at least provide plausible deniability if you are smart enough to keep your mouth shut about religion on the campaign trail.

Unitarianism, obviously, is one of these. If I were ever to run for office, I would simply join a UU assembly and leave it at that. This would probably restrict me to running as a Democrat, though. If I wanted to run as a Republican, I would have to be more careful, and say "I was baptized in the Methodist church".

If Zuckerberg wants to give it a go as a Buddhist, secular Buddhism exists, and certainly no one on the west coast would bat an eye at it, but the other regions of the country would probably either have a problem with the religion itself, or that Zuckerberg would be practicing it as a white guy.