Can't do that. Zuck can't use FB resources or any form of support from FB (other than employees who volunteer outside work hours) for a political campaign. If he does he'll have to reimburse the fair value of services received from campaign funds.
But what if it were done on behalf of a super PAC that just happened to be in favor of his campaign?
It would be legal as long as there is no coordination with the campaign but I can imagine enterprise ending class actions against FB by minority shareholders if the Board violated their fiduciary duty to shareholders by involving FB in the campaign.