Yes, but they will be also bookmarking a lot of stuff that only they bookmark.
In our dataset of over a billion bookmarks, 80% of urls were only bookmarked by a single user. These urls comprise about 50% of all bookmarks (user-document pairs).
Incidentally, worio.com (my startup) offers full-text search of your bookmarks (though not a viewable cached copy, like these services).
I'd bet the %age of external resources referenced by bookmarked pages that were unique to a user would be a lot lower... which is where the aggressive de-duplication would come in handy.
In our dataset of over a billion bookmarks, 80% of urls were only bookmarked by a single user. These urls comprise about 50% of all bookmarks (user-document pairs).
Incidentally, worio.com (my startup) offers full-text search of your bookmarks (though not a viewable cached copy, like these services).