#13 del.icio.us

When someone asked me about why they would use del.icio.us rather than normal folders within bookmarks on the computer, I was able to tell them that del.icio.us is transportable and not confined to the individual computer or login you are currently using. (I think!) There is also the 'social' aspect, although I think I would only use that to glean others searches rather than wanting to make actual contact with them.
I think the bigger these sites get the more structure they require with folksonomies. There is already the facility to edit and merge your search tags and to a certain extent the social tagger will be influenced by the tags that have used before.
I thought that the Commoncraft YouTube video on 'Social bookmarking in plain English' was a very useful introduction.