Yes, and, just a guess: Were I on Team Zuck, I'd have every competing messaging app installed, just to see what everyone's doing. eg I'm certain everyone touching Instagram also uses Snap.

Further: LAN Manager era Microsoft had a Novell Netware group, Microsoft also continued to use (Windows port) of sendmail while selling Exchange, had an entire Macintosh enthused business group.

TLDR: Of course Zuck has a Signal account.