I think ksherlock is wrong. Marco explained somewhere that he doesn't store for a url the content only once (like the url serves as a hash), but stores the content of a page for every account uniquely (his database is somewhat 200 GB). I also did find this:
http://news.ycombinator.com/item?id=2336332
> Instapaper honors access. If you have access to that Article it will save it, if not then you won't get it.