These Social Networking and web-media sites that, for the most part, were "fixed" when they weren't broken, or just were way too damn slow! I can remember a MySpace page taking 55 minutes to fully load on broadband, after the service allowed users to edit their own pages and inject all kinds of horrible templates, features and other nonsense that choked the system!
Cause of failure: Bloat, bandwidth hog, lost user-friendly edge to Facebook.
(2) Technorati - succumbed to stupidity after declaring MySpace pages were blogs - could have been as big and as influential as Google! (these guys get the "I really blew it" award!)
(3) Entrecard collapsed after years of neglect and system-gaming by cheaters in Malaysia and Indonesia
(4) MyBlogLog grew too big and then fell victim to mismanagement by a company that didn't understand it
(5) Friendster - slow-loading bandwidth issue became Achilles heel
(6) digg - outlived its usefulness, went the way of the horse and buggy.
(7) Xanga - failed to provide users with tools and features every competitor offered
(8) Orkut - again, bandwidth: queen of buffering graphic
(9) eons - one of the stupidest ideas ever
(10) ping - people have their own ideas about music and this thing was described by one teenager as "alien."