When I search in Google and moving the mouse over links I see the link of the current page like in this picture :

But when I open this link and stop page loading to see the URL I find this
It’s used by Google to sotre data about visitors of this link or when a user copy the link he’ll get just the right URL .
The same thing with Facebook for external links ( l.php )
This option help user to see the whole address.
I like to use it in my website so how can I do ??
This is how if works in IE. I did not try other browsers, it is possible they are using the same or similar method.
The sample below illustrates what happens:
The link here points to http://stackoverflow.com, and when you hover mouse over it, it displays http://stackoverflow.com as a target of the link. When you click on the link, instead it brings you to http://www.google.com, because that piece of javascript overrides the
hrefattribute.