I don't get it.
We have all manner of Web 2.0 properties to cater to just about every sort of online need. I'm not going to name any specific site--any such would be either completely obscure or wildly controversial--but you know what I mean.
However, bookmarking seems to have remained a backwater. There are apparently a lot of sites that are connected with bookmarking in some way. (See, for example, the bookmarking category on this list.) However, the best one can say is that no newcomer has gained any real traction and the sort-of-known--at least within the geek crowd--have done remarkably little over the past few years. In fact, I'm struck that essentially nothing has changed since this 2004 James Governor post. 3+ years is an eternity in Web 2.0.
A del.icio.us 2.0 is in preview; perhaps that will make this discussion moot. The oddly-named del.icio.us certainly appears to be the best-known and have the most critical mass of the social book mark sites. It's just that it hasn't changed in ages. (It's a Yahoo property, story ?)
From my perspective, the social aspect of these sites is almost secondary. Yes, there are a few friends whose bookmarks I keep an eye on. And, when tagging, seeing what the "crowd" has used as tagging terms can help you stay consistent. But I don't view the storage of bookmarks as primarily a social or sharing activity.
I mostly use del.icio.us to store bookmarks for my own use and to generate blog posts such as this one. Today, that means dealing with homemade scripts and a strictly limited number of characters in the comments or notes about a link. Nor does del.icio.us provide any real organizational tools to easily consolidate or change tags.
In short, bookmarking is such an obvious "cloud" application; a bookmark isn't much use without an Internet connection. (Permanently saving the content of pages is another topic that I view as largely independent of this one.) Yet it's an application space that has been poorly served by "Web 2.0" to date.