Push notifications in social networks?

Question

PHP

For example, on sites such as vk.com, badoo.com, when you receive a message or a like, a notification appears.
But if you look at the console, then there are no web socket connections or ajax requests.
How are these messages sent?

Answer 1

no ajax requests

There is. They even have documentation about Long Polling servers.
https://vk.com/dev/using_longpoll