How do real time updates work?

Stack Overflow is using Web Sockets for real time updates. If you take a look in the source code (line 35), you would see:

StackExchange.ready(function () {
    StackExchange.realtime.init('ws://sockets.ny.stackexchange.com');
    StackExchange.realtime.subscribeToInboxNotifications();
    StackExchange.realtime.subscribeToReputationNotifications('1');
});

But note that some Opera versions does not support WebSocket. (not until Opera 10.70)

However Facebook does not seem to use Web Sockets, and I think they are just using simple XHR with a technique called long polling, which the server holds on to the connection until there is new information, and then the server will response to the request. If you open up the developer tools you can see that there is always one request which has a status of pending.

It is indeed, sending a request every ~60 seconds.

It seems that Twitter also uses simple XHR (1 minute intervals) for their "real time updates".

Facebook uses long polling/Comet. So it makes a connection and waits for a response, if no response, then it times out and tries again. The timeout is around 40 secs. That's how it does most of the instant updating. However they use a combination of techniques. More on long polling here.

http://en.wikipedia.org/wiki/Comet_(programming)