AJAX can be bad SEO and sometimes AJAX can increase server loads, especially with long-polling. However, HTML5's WebSockets, would help improve performance in this department. But if you're looking for something like Facebook, you'd need to run a real-time server like Orbited (orbited.org), or Tornado. Honestly would love to see a forum system made to use the real-time system but i think its a few years too early to be hoping for it.
I have written some basic Apps that use such servers and I can tell you its quite lovely, i'm looking into HookBox (hookbox.org) too.