Snapchat worth $10 billion? do you think this because turned down a $3 billion acquisition offer from Facebook last year?

the number of users sets the value.

Snapchat is a promising app, it has an enormus users base which give it this high value, just like Whatsapp app, as long as its popularity increase, its value increas correspondingly. 

Sapchat is a great chatting application with features that makes it better than what'sapp, but whether the application worth that amount of money or not that will depend on the number of users using it.Mainly the large companies like google tends to buy such softwares to have their clients too.

may be !!!