Check this out! http://www.fring.com/
What framework did they use?

And what frame work is easy to implement a full ajax page such as youtube vedio play page?

I can't really tell what framework their using. I would assume either PHP or ASP.NET MVC.

For me Ajax is easier with ASP.NET, but that's mostly because I learned Ajax through .NET.

Use view source and you should see .php links... If you try to enter an invalid URL, you would see a link to support with .php in it.

view source doesn't show link extensions, just paths, which is why I mentioned ASP.NET MVC.

If you go to their support page, then view source, one of the links has a query string on it, and has a .php extension, so it's PHP based.