I think what will get most investors worried is how WhatsApp is able to make back that money.
For what I understand Whatsapp charge 0.99 cents on their per pay service,
which current 300million users only they might make 300million dollars, but 19billion?
There is no adds or anything like that in whatsapp from what I understand.
They might develop it further since Mark Zuckerberg fail to purchase SnapChat
What I hear from bloomberg this afternoon is that Only 2% of americans uses Whatsapp, most of them just prefer to use text messages. But in Europe, South East Asia, users are up to about 80++%
China on the other hand has a very low user rate as well due to WeChat , and Korean they have their own Chat App as well.
So based on the 2% of Americans and with FB's publicity on Whatsapp, there is a very high chance it can grow.
And India with its high population uses whatsapp a lot.