$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.
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.