“When you use Signal, your data is stored in encrypted form on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.”
That has not been the case from what I’ve seen (I’m Australian). The only widespread methods of communication I see are SMS and iMessage. Things like Discord and Instagram are only used among younger people.
Edit: Actually people do use Facebook messenger. Don’t know how many though