You want to see how much eyeballs are worth, look at reddit. They are pretty much a corporate charity, with their reddit gold scam going on.
When people get accustomed to not paying for things, they don't pay for things. I'm not going to pay for a newspaper when I can read it all online. And yet, I'll happily go to dinner one night a week and spend more than it costs for an entire year's subscription to the local newspaper.
WhatsApp is $1 a year. That's great if you're running with a few hundred employees and have such a massive user base. That's doing more than okay, in my book. But $19B? You're not getting that back. Certainly not in any timeframe that matters. Because what's hot today will not be what's hot 5 years from now. We all know that. It's how the tech industry works.