True, so people are linking instead of embedding. But the point is the same... If one person makes the decision to link to scribd instead of the pdf, and 100 people follow that link, it doesn't really mean 100 people value the service.
True, although I'd guess a fair amount is webmasters trying to cross promote their content. Seems like if you own a porn site, you may as well post some "tasters" to scribd with your web address on, etc etc
That's true and so was your original argument, and both are true of YouTube as well. That doesn't make their traffic any less impressive. It's just using good viral spread mechanisms.
They are quantified, and quantcast shows the vast majority of their traffic is to their destination: http://www.quantcast.com/scribd.com
see scribd.com vs The Scribd Network