According to this tweet, this shows the Zuck himself uses Signal: https://twitte...

johannes1234321 · on April 4, 2021

You must observe the competition and maybe he doesn't want his employees to be able to see what he is doing :-D

Yajirobe · on April 4, 2021

Yes, that is totally Mark Zuckerberg

bellyfullofbac · on April 4, 2021

What a useless screenshot. Sure if we believe him then he actually added Zuck's number into his address book and he got this notification from Signal. But if I want to doctor a screenshot like this, I can rename my non-Signal-using friend in my address book to "Zuck", and make my friend install Signal, and voila, "Zuck is now using Signal"...