With comments like this, I can understand why he would need personal security. I feel like this rhetoric is dangerous. It sounds to me as if you might be suggesting this sort of "solution" has some legitimacy. I apologize if that's not how you meant it.
I think they're being reasonable. Have a sense of history and of scale. Wars have been fought and millions died for issues much smaller and amounts of money much more meager. When you get to a certain size, when your actions literally affect over a billion human beings, and when there is more capital tied up in your venture than in many small nations... why on earth would you expect people to suddenly not treat you the exact same way nations are treated, up to and including armed insurrection? I don't support such a notion in Facebooks case, but I do think it would be wise for anyone dealing with numbers that big to consider history. Has a war ever been fought over the amount you are dealing with? Are you doing something a war was fought in the past to stop?
I don't think he's suggesting anyone should do anything violent. I think he's suggesting that if Zuckerberg continues down this road, he's going to face problems by whatever routes remain available. The only thing GP said anyone should do is Zuckerberg should "probably hire more personal security".
Would it have come across as less of a threat if he had quoted JFK ("Those who make peaceful revolution impossible will make violent revolution inevitable")? Was JFK threatening Latin America when he said that to their diplomats?
This is a very common rhetorical tactic for people who wish to advocate violence. Parent comment is not advocating violence by asking someone to commit a violent act, they are advocating violence by arguing that violence is reasonable / inevitable.
Crime exists. I advocate that everyone apply whatever measures that fit within their means to ameliorate it, or avoid it altogether.
It applies on the small scale as easily as the grand. If you insult someone's mother, watch for and avoid the retaliatory punch. If you violate the privacy of a billion people, call them morons for trusting you, profit greatly from their personal information, and continue to demonstrate eroding ethics, then install a safe room in your house(s) and hire some bodyguards.
That said, individual violence is reasonable, after the non-violent and collectively-violent possibilities have been exhausted. There always has to be some way to effectively retaliate against anti-social behavior.
It is not "legitimate". That word shares a root with "legal". But we should not pretend that some people do not do illegal things when the stakes are high enough.
Rich people get kidnapped and held for ransom to fund rebellions or criminal cartels. It's a thing that happens. Even heads of state get assassinated from time to time. It's a thing that happens. At the level of super-rich that Zuckerberg inhabits, I'd guess he already gets at least 2 attempts at theft, robbery, blackmail, or extortion per year, and some of them might even be successful. It's a thing that happens. People don't like to talk about it, because the successes encourage further attempts.
There were even GTA 5 missions entirely about manipulating public stock prices by criminally compromising or killing a company's CEO. That's how the player can make all three of their player-characters multi-billionaires.
I'm simply saying that if the only way someone can get to you is via criminality, then that is the way someone will get to you. Most people would always try the "legitimate", peaceful option first, if there is one. That's the primary reason why people choose to leave themselves open to the law. Surrendering to the marshals or getting arrested by state professionals is a more favorable alternative than getting caught by an angry mob. Just ask Qaddafi.
It's just engineering. If your structure is designed to fail at one weak point, then you can monitor that possibility more carefully, and predict the behavior following that particular failure. You can purposefully put in a weak element, so that if the structure collapses at all, it will do so more slowly and in a safer direction. In a corporate structure, you allow yourself to be removed by majority vote, and the failure mode is that you get paid off and live on in semi-retirement. Remove that option, and you don't know how you'll leave your office. It might be a memento-mori-scale heart attack, or complete failure of the company, or a sudden black screen as "Don't Stop Believing" plays on the jukebox.
In the wake of the vote to remove him, Zuckerberg could either step down voluntarily, or increase his security. He won't do the former, so he should do the latter.