Del.icio.us is a great tool. It's especially great when you hook the RSS feed up to your feed reader because you can peek in on the sorts of things your friends are bookmarking. Unfortunately, for being such a “social” site, there does not seem to be a good way to build up your social network. For instance, on LiveJournal, I can see who has subscribed to (or, to use LJ nomenclature, “friended”) me and can reciprocate that. I can see the people that my friends are subscribed to, which helps to determine the user names of people I may happen to know in real life, but do not know their user names. Del.icio.us (as best as I can tell) does not have an equivalent feature, so I have to flounder around trying a bunch of different user names that may or may not belong to people I know to see if they are linking to the kinds of things I would expect those people/usernames to link to–or I have to remember to ask people when I talk to them.
In an effort to maximize laziness and slack, I now ask everyone at once what their del.icio.us user names are. (Mine is BriEnigma, in case you were wondering.)