This has been extensively reported in variety of media.
Origin comes from this 2016 coverage [1]:
In a Facebook proxy statement published by the Securities and Exchange Commission (SEC) on Thursday, the company paved the way for Zuckerberg to one day take a "voluntary" leave from his post as CEO and serve "in a government position or office."
Followed by his New Year Resolution [2] and state of affairs in the country, the public started pressing that narrative which Mark replied to [3]