That's correct for many websites, but others such as Facebook have such a complicated but still correct UI on a desktop that it's appropriate to have two different websites.
Nope. That was true about a year ago, but not anymore. Facebook UI on a desktop is horrible. Try using it with 1024px wide screen - most apps/games aren't fully visible because they give 760px to the canvas and the rest of their UI is not responsive - so either the ad space on the right or the content (app/game) has to suffer.