There's no pre-requisite to running a billion dollar company that says you have to be charismatic. Heck, just look at most billion dollar companies that aren't called Apple. Zuck should do what he's comfortable doing.

If I were him I'd consider putting forth a different 'face' for my company simply because he can afford it, but I don't respect him any less for putting himself out there to represent his own thoughts and aspirations.