http://news.ycombinator.com/item?id=834352
http://news.ycombinator.com/item?id=832142
There's already a great deal of discussion on the first of those, so it might be worth reading that before commenting here.
FWIW, I think that what scribd does is a Bad Thing(tm). I can image having to check every week whether something I've written and copyrighted has appeared on their site, and I think it's unsupportable that they say "It's your problem to check - tell us and we'll take it down."
Perhaps it would be interesting to require that any money made from having my work on their site must be transferred to me with interest. Further, to require that such accounting be made auditable. If they make money while my work is on their site in breach of my copyright, why shouldn't I get it?
Still a nightmare to enforce. Every author constantly having to check to see if their work has been pirated.