I don't see Pinboard mentioned yet - pinboard.in
- and recently a competitor https://historio.us/ apparently some people are moving to.
Pinboard has some nice features like RSS feeds from bookmark tags, and they also have an archiving feature that archives all the sites you bookmark (for some additional $$)
I use Historious because I wanted a free solution that was similar to delicio.us which I had used for years. It's easy enough, I use a bookmarklet to add pages.
I think historio.us and diigo are pretty much the same, no? I don't need pretty visual representations of my bookmarks. Just cross browser sync, ability to quickly search & find.