How do companies like WhatsApp and Hike earn money?

In the case of WhatsApp they probably use revenue from the apple affiliate program, this is how it works:

After you sign up and are accepted you can generate links. When someone clicks on the link a cookie is installed on their iDevice, this cookie attributes all purchaces generated to the owner of the affiliate link for the life of the cookie (24 hours). The owner will then recive a 7% commission on those purchaces.

It's all about the data

A lot of modern social apps (and non social apps) do not generate any ad based revenue. They do however collect a lot of data that is valuable to them or other companies. Think about Microsoft buying Skype or Facebook buying Instagram. Having these services allows the purchasing company to have data that they can then use to build other targeted products.

That's not to say that every start up has the same exit strategy (hoping to get bought out). Many startups have one or two founders who tend to get lucrative offers from industry to build similar or more tailored versions of the same product.

Remember that for every startup you see get famous and turn in to a bigger entity, there are several others who had potential but never made it big (and those that invested in them lost out).

Whatsapp Application is either free or a paid application for iOS users, depending on the region. In India they charge $1 at the time of downloading the application. In the US, it's free for the first year, and then a $1 per year subscription (though the initial app download is free).

This is one of their revenue sources which I know of.