What not to do with AJAX

One of the features of AJAX will be to "post" smaller data than a full HTML page.  While XML is very verbose, it will be better than sending a full page and all of that overhead (Ex. ViewState in ASP.NET).  I am fearful that  with that, developers will take the opportunity to go back around to the server in a relatively small amount of time and use AJAX for everything.  Now imagine that you are a public web site, you have a couple thousand users using a chat program built in ajax that sends data to the server every few seconds.  Depending on network bandwidth and server CPUs, it may overload the web server.  I don't see a "major" issue in going back and forth to the server on an intranet application, but I can see major issues when running a public web site and doing thousands (or more) AJAX calls over the public internet.

Wally