I am currently working on what del.icio.us could have become if it wasn't acquired by Yahoo (www.shelfbeta.com), I want to hear from others why they think it failed.
Has Delicious failed? It might not be flavour of the month anymore (since a long long time), and I don't know anyone who uses its social features. But as a way to quickly access my bookmarks from any device, it still has a job and it does perform it well. I like to be able to access my public links without having to log in.
What is a partial failure, is the flat redesign. Very nice to look at, but slightly infuriating to use, especially when adding a new bookmark.
Del.icio.us was a Good alternative to old Bookmarking system. I don't know the current reasons they've killed but I suspect many of them related to non-profitable platform, heavy use and management for sure.
Later, it clearly became unrelevant because right now all links are seamlessly synced between devices without having to enter a website, right in the browser. I guess the only one who doesn't is Firefox, but the rest they have their own.
What is a partial failure, is the flat redesign. Very nice to look at, but slightly infuriating to use, especially when adding a new bookmark.