Decaflon

Facebook has the client open a persistent connection to the IM server and uses long polling to send requests and continually get data from the server. Long polling is a mixture of client pull and server push. It works by having the client make a request to the server. The client connection blocks until the server has data to return. When it does data is returned, the client processes it, and then is in position to make another request of the server and get any more data that has queued up in the mean time. Obviously there are all sorts of latency, overhead, and resource issues with this approach.