They charge $1/year/user, though the first year is free.
One year ago they had 200m users. That number has not shrunk.
There is a free year, and then it seems like "everyone" gets a free year renewal.
I would not be surprised if 10% of their userbase or less had ever paid. Which is still a good number, but not the one you make it out to be.
Revenue is a part of it, but also remember that it's only a proportion of generated value. The fact that so many people use the app to communicate indicates that it's providing them with substantial value, even if the company chooses not to capture more of that value.