It is described in Sarah Wynn-Williams’ book. According to it, Zuck even battled to change the stock structure in order to take 2-year hiatus from CEO duties:

  …without losing control of the company if his absence is “in connection with his serving in a government position or office.” I assume the two years would cover the period Mark would be taking on the—presumably time-consuming—endeavor of campaigning to be president of the United States.

Never happened. Zuck considering running for president was just hearsay.

Surprised that people are still quoting a decade old rumor.