At 400M users, how much more are they really going to grow? Maybe 2-4x. They're at a point where they're almost bounded by the number of people on earth with mobile devices.
Well, given that there are roughly 7 billion people on earth, and the number with mobile phones is currently 4.3 billion, that means WhatsApp is at 9% penetration of a still growing market. I think they can grow a little bit more than you estimate, and regardless, 4x of 400mm is an increase of 1.2 billion users.
There's only about 1bn smartphone users, of which Whatsapp probably has 45% penetration, and some other large percentage are on competing platforms. Yes, that 1bn is growing, but it's also moving into market segments with much lower income levels. Even if we think this is going to increase 4x that probably doesn't justify a 12X sales valuation.
That one billion number is not static. And it has a lot of room to grow.