Even more directly, it looks like Congress passing a bill they know won't get past a veto. Rather than the uncertainty of Brexit, there's a single voter with veto power and a pre-declared position. (Though this is more extreme; in voting-theory terms, Presidents don't even have proper veto power, but Zuckerberg is a dictator.)

I think the better headline here might have been "percentage of outside investors voting against Zuckerberg rises 17 points". You're exactly right - it's a show of no confidence, but hardly a guarantee that these voters actually want him removed.