The problem with all these web analytics is that they usually fail to take into account API usage-- I wonder what percentage of the del.icio.us userbase uses the FF extension or other widgets to use the service?
No. Not being able to find stuff you previously saved quickly is what killed it (at least del.icio.us). Tags, at least for me, simply aren't good enough.
I just have a "to read" page in my personal wiki where I can copy and paste a link in about 5 seconds, and I don't need to type a title. I have fulltext search which makes up for the loss of tagging. And plus I don't need to type a title.
The participation threshold is too high. I was just talking to a former heavy del.icio.us user who only uses their front page now. Users always seem to prefer lower-friction services even if they're not as rich in content.
