Power is not measured by “users.”
What actions do you think Zuck could take which would demonstrate that he has more power than Trump?
And, why do you raise the hypothetical of Trump shutting down Facebook? In particular, it's my understanding that many of Trump's foes are blaming FB for his win.
Nukes are indeed powerful, but media organs have started wars before.
Remember the Maine!
If yes, then Facebook must necessarily be powerful by the same metric.
If no, why not?
In any case, even if the MSM effectively shapes public opinion, the POTUS commands the armed forces.