It's not exactly shunting vast amounts of data, I'd think a buck per year per user would be pretty good for text messaging.
Given that it's unlikely users will jump ship if they are happy with it over $1/year and the fact that they have 400 million users I think there's a pretty good chance that revenue number will come up.
...but there are so many options to send text messages for free. Why pay the $1 / year?
Privacy and no ads. The "free" alternatives are not "free".
Telegram is "free". I'm not sure why "free" needs to be in quotes. I don't know how I feel about Privacy being touted as a benefit of using anything owned by Facebook.