The fact that probably millions of people would vote for him to be president is terrifying to me.
Zuckerberg strikes me as hell-bent on global domination, but doing it with the most plastic smile possible. Given any sort of real authority, I doubt even humor at his expense would go unchecked.
We've had numerous celebrity politicians before Trump. Franken, Reagan, Arnold, Ventura, Springer, etc... The list continues[1] and it definitely wasn't Trump that was the first (although he holds the highest position of any of the other celebrity-actor-politicians, without having first held another political position).
[1] https://en.wikipedia.org/wiki/List_of_actor-politicians#USA
EDIT: Clarified my comment on Trump being the "first" celebrity-president