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.
It certainly needs more love - another problematic part is when you have messages open but you want to see stuff on the column to the right of the newsfeed. Its design leaves some to be desired.
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.