I am trying to understand how Facebook's chat feature receives messages without continuously poling the server.

Firebug shows me a single GET XmlHttpRequest continuously sitting there, waiting for a response from the server. After 5 minutes, this never timed out.

How are they preventing timeout?

An AJAX request can just sit there like that indefinitely, waiting for a response?

Can I do this with JSONRequest? I see this at json.org:

JSONRequest is designed to support duplex connections. This permits
applications in which the server can asynchronously initiate
transmissions. This is done by using two simultaneous requests: one
to send and the other to receive. By using the timeout parameter, a
POST request can be left pending until the server determines that it
has timely data to send.

Or is there another way to let an AJAX call just sit there, waiting, besides using JSONRequest?