Delicious Alternatives
Disclaimer: I have only used Xmarks and Google bookmarks on this list. I love the former and can't stand the latter. Also, I tend to be a private person about my link links, so there may be some secret and amazing social aspects to Xmarks and Google bookmarks that I just never discovered. These are not the only possible sites out there, but they all seem stable and enduring. For the other sites I've relied on the websites and off-site reviews.
(1) Xmarks
Pros: Very easy to use cross-browser sync, ability to share only certain folders with others, stable with active and dedicated user base, provides link ratings and reviews on Google, links are also accessible online, Firefox & Chrome add-ons, can create different profiles for home/work/etc computers
Cons: No public API, very weak social aspects (you can access the ranks/reviews and add reviews, but not much more), del.icio.us import only takes your 100 most recent links
PC World Reviews, Think Digit Review (Mentions the newly implemented premium features; the premium features though are more in thanks of support than anything else, imo, Ars Technica Review
Tags are the issue with Xmarks. If the tags show up when you import your del.icio.us files to Firefox/other browser, they they should also sync with Xmarks. Some people have had trouble with this, though. One workaround is Transmute, but I'm not 100% sure how that would work.
(2) Diigo
Pros: Can sign in via FB/twitter/others, easy annotations, can create groups, stable and popular, Firefox & Chrome add-on, can save and annotate webpages as well as links (so, like evernote+del.icio.us, maybe), provides a way to import from del.icio.us
Cons: Uses a toolbar, some people have had trouble with the import feature
CNet Review, App Appeal Review
(3) Mister Wong
Pros: Can sign in via FB, image oriented, popular in Europe, multi-lingual support, Firefox add-on, includes an import for del.icio.us bookmarks
Cons: No public API
Mashable Mention, App Appeal Review
(4) Faviki
Pros: Semantic aka common tagging (tags are same across accounts), an up-and-comer, provides an easy import from del.icio.us
Cons: Semantic tagging (tags rely on wikipedia/dbpedia/etc entries), not very fan-oriented
App Appeal Review, ReadWriteWeb Review
(5) Google Bookmarks
Pros: Stable, easy to use, doesn't require opening a new account anywhere, links accessible online
Cons: Little control over how links are imported, very weak social features (you can access public lists, but little else)
If you have more, please share! Also, if you use or have used one of the above, please share your experiences.
Note: More information and lists here on LJ courtesy of r_becca.
Other Sites
Pinboard (not free, price rises with size of user base)
Histori.us (free only for the first 300 links, but tags will stay in-tact)
ZigTag (semantic tagging)
ZooTool (open api, also newer and so you'd definitely not want to make it your only back-up)
Evernote (more designed for web clippings than managing bookmarks, even premium accounts are limited in the amount they can save per month (1 GB))
BlinkList (another 2010 newbie)
Connotea (academic, not good for fannish links)
Zotero (academic, not good for fannish links)
Mendeley (academic; not good for fannish links, but does include a social network for papers and research sources)
Instapaper (saves links for reading later, but may not be suited for large libraries of links)
Faves.com (open API, not sure how the import work)
Transmute (basic conversion level is free, bookmark management is not)