[adventured]

If WhatsApp can ultimately generate $1 billion per year in net income, it's worth ~$19 billion.

It's extremely likely they can do that, if they can hold their market position. The business itself is capable of being very profitable over time, and has an extraordinarily low per user cost to operate.

It's not likely WhatsApp will ever be a wildly profitable purchase for Facebook, but it doesn't need to be. Half the point was to eat a rival, the same reason they bought Instagram.

The likely worst case scenario right now, outside of WhatsApp imploding, is that Facebook overpaid by 1x. To grab a billion users, and fend off a huge potential threat, that's easily worth single digit billions of dollars to Facebook.