$19B is actually a really reasonable price for Whatsapp if you're FB, though it wouldn't be the same for other buyers. Whatsapp not only has a great business, they posed a direct threat to FB's business. It's a large price/earnings multiple but it's really not outrageous considering.
Edit: responding to your edit:
> My point was that FB wouldn't pay $19B unless they planned to monetize beyond the $1/user/year model.
Maybe. I think it's more likely that the social industry has started to peak, and that we'll continue to see FB at the top, buying off small companies as they become threats. Take a look at the movie industry, or any industry with a traditional "Big 4" group of companies.
In other words we can go with your price analysis or with the actual number paid for it. I'll go with the actual number.
A simpler example: Is this bottle of wine worth $200? Maybe not to you...You can make fun of that price. That's cool, free speech, say what you want. But it is selling at that and someone keeps paying that amount for it.
[edit] Seems my comment is being misunderstood. My point was that FB wouldn't pay $19B unless they planned to monetize beyond the $1/user/year model.