How to notify mobile clients about events on the server?

Question

Mobile development

Good afternoon.
There is a site on asp.net MVC + web api. And there are applications on different mobile platforms (android, iPhone, wp). Each mobile client logs in with his own login and communicates with the site via json web api. The site has a form with which an order is sent for a specific client. How to notify a certain customer that there is an order for him?
Of course, you can make an api method that will check for the client if there is an order and pull it with a certain periodicity. But if there are thousands of clients, I think this method will be very expensive.
Interested in how it is architecturally done correctly. For example, as it is done in the Vkontakte application. As soon as you receive a message, the mobile client notifies you immediately.

Answer 1

Push notifications:
Apple
Android
WinPhone

Answer 2

There are also sockets, for example. An excellent solution, and the benefit is enough.