Assuming many or most here think it is relevant, it should be possible to respond to my question reasonably, without buzzwords and with only moderate downvoting.
1. Before browsers could sync my bookmarks across multiple computers, posting them to delicious was the best way to have access to them anywhere.
2. Before instapaper let me save articles to read later, Delicious was a great way to have a tagged backlog of things to explore whenever I have freetime.
3. Before Twitter, the best way to know what your favorite developers or designers were thinking about was to follow what they were bookmarking on Delicious.
4. Before HN, looking at the usage of tags and stories per tag helped me figure out what technologies or topics were growing in popularity and find out what to read first about them.
That's how the theory goes, anyway.
If the above analysis sounds dated, that's because it is. Conceptually, Delicious is really an app straight out of 2006. But like Flickr, it's one of the handful that's worth having around in more or less original form.
The key to grokking Delicious btw is that it's not for "bookmarks" like the ones you probably have on your Bookmark Bar in Chrome right now. It's for articles or links of interest that you might want to reference later. Presumably there are other services that do or claim to do this better, but I have yet to find one that's stripped down to the same level of usefulness as Delicious.
I've never really cared what others are bookmarking, as I've found Google has traditionally been just as good as a big group of anonymous people at telling me what to look at.
Cross-link that with people I follow on Twitter and it might get interesting, though...
And of course it syncs across Firefox installs, including my Droid.
To me, Delicious is like a really organized attic or "backlog" of interesting stuff I found online and want to remember for later. It allows me to find things back because of the tags, but it doesn't take as much time as writing blog posts and I don't have to think about burdening my followers the way I have to with Twitter. I actually wrote about it a while back: https://micheljansen.org/blog/entry/614 (see comments)
I don't use the social features at all and I am really curious what direction Delicious will take under new management.
Only half joking, I have 134 tabs open.
People like to be building something. Delicious is less edited than the wikipedia but it's still a massive datastore, semantically tagged, and filtered by its users. And if Delicious would get off their cans (and it looks like they will) it could be the single best way to share a link.
Example: I write software for, and have a strong interest in, professional photographers and I keep bookmarks of my favorite photographers for inspiration. Many of them were found via delicious.
(2) is very interesting because "rating" on Delicious is not a vanity contest (e.g. Facebook) and people aren't rating because they "agree" with an article (e.g. HN / Reddit).
Rating on Delicious is implicit, the action itself (bookmarking) serves another direct purpose to the user; people are bookmarking stuff when they want to return to that article -- much like how Google's ranking algorithm works. Yes, it could be gamed, but it's not a high-priority for spammers yet, and I still find Delicious to be a very low-noise source for interesting stuff (much, much more useful than HN; basically I come here for the comments only).
And as an example, searching for Ruby gives you plenty of tutorials in the very first results, searching for "Google" gives links to their interesting assets (like their Fonts directory) -- people are more unlikely to bookmark flames / yellow-journalism; as that pollutes their own database of links.
I saw a comment saying it's an app from 2006 -- well, if that's true, 2006 was a lot better than 2011.
The social part is, I think, less interesting but occasionally handy (ex. here are all the interesting things I've read about javascript recently: http://www.delicious.com/tlianza/javascript ) and I think the aggregated data could be interesting, but as Digg's woes have shown, it's proven challenging to make a compelling experience around it.
back to the community...
When I need to find a good link or resource related to the web design/developer market searching delicious shows me what other people have bookmarked. It's like Google but only results which have been liked by users, so much so that they wanted to save it for later.
I still think that's one of the most powerful aspects of this small-ish community. The problem lies when you scale it, as I have no interest in searching the bookmarks of the CNN/Facebook/YouTube crowd.
Kind of like why Digg was so great. In the beginning it was news submitted and voted on by geeks like me
A couple of things lead me to gradually stop using delicious.
1) There was a horrible update for their FF plugin a couple of years back that essentially broke the way middle clicks worked (and the way I preferred to use it).
2) My friends stopped using it so much, and so it became gradually less useful for me as well. I still check out the /popular page for things I'm interested on occasion, but the utility from my friends bookmark feeds is almost gone.
I never used the social aspect much, but the public aspect is wonderful. It's a fantastic way to find related sites.
I've been using delicious (or similar) for at least five years. My first reaction when people ask about it is sort of the same reaction I feel when people say "I can't understand why anyone would want tabbed web browsing; can't you just open lots of browser windows?" :)
My primary use for google these days is searching sites, like this one and stackoverflow. If I want to find an unknown site based on a keyword, delicious guarantees me the best results most of the time.
http://delicious.com/popular/biology http://delicious.com/popular/lisp http://delicious.com/popular/finance
To me, Delicious is more valuable than Facebook and Twitter. Social bookmarking lets us to extract information from the chaos.