What technique facebook use to notify users?

I'm just being curious. Does anyone know what tech开发者_Python百科nique/how facebook notification works? Because, when ever someone post a comment on my wall it appears in the notification, it happens instantly. It looks to me, server sends notification to client instead of client dialling to server every such amount of time.

Any idea anyone?

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Well, their online chat uses Comet (via MochiWeb) so I wouldn't be surprised if notifications are handled in a similar way.