Chat with node.js always on top on all pages

I am building a person to person chat using node.js in a php website (using zf) I am wondering how facebook (for example) is doing to always have the footbar visible even when navigating from pages to pages. I first thought they used ajax everywhere so the page is never realy reloaded but it appears not as you can use the browser back/forward buttons to navigate (and I don't realy see any ajax call in my browser developper tools). So, my second thought is that all facebook is build with a kind of node.js alternative so everything works with js sockets ?

Any idea how to do a similar system, to have my chat windows always visible even when navigating my website?

Thanks