Stats like that are private not because they might encourage competitors to enter your space, but because they might give your existing competitors ammunition to use against you. Very obvious example: someone who is considering becoming your customer may chose a competitor if they know that the competitor has three times as many users as you.

Or maybe you're doing really well, but you want to save the fact that you have over a million users and use it up get a great press story at a strategic moment.

There are plenty of good reasons to keep your numbers private.