I remember using XMPP for a site I built that connected tenants and landlords, a reverse MLS of sorts. Back then, I choose XMPP because I had heard this is what Facebook was using at the time for their new messenger and it provided services like online status, typing status, and live messaging out-of-the-box. I don't recall it being terribly difficult to implement and it was awesome to see it working for the first time!

This was almost 10-years ago now, it's incredible to see this technology still moving forward.