Nothing of what I have written above applies to these 3. Neither can be written by a 14 year old over a weekend, YouTube is a lot of tech to handle. Flickr never nad problems with users paying for their services.
Finally, del.icio.us does not even belong to a group of companies this thread is all about (over-hyped startups with venture millions, producing crap) - it was a personal part-time pet project of a guy who didn't live in the Valley, didn't ask for millions for coding it. He wasn't even a professional programmer as far as I know.
In my view, you can be either a technology company (YouTube) or a revenue company (MySpace). Or both (Google).
Overwhelming number of Web 2.0 startups are neither.