Due to the sandbox on javascript calls, you can't do this (you can't call outside the domain the JS was loaded from), at least not directly
There are 2 methods around this.
The first would be to load the requested url in an iframe rather than a div by setting the src parameter using JS. Simple, easy but would limit your access to the data in the iframe.
The second would be to make an AJAX request on your server and your server then looks up the URL and returns the HTML content (pretty easy to do with CURL or similar). This lets you play around with the returned content a bit more, but again due to the sandbox on cookies you won't be able to request anything like a users facebook page or anything that requires a session as it would be working through the servers session and not the browser session.
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