I agree that it's disappointing that the article didn't mention reddit, but the compete.com data doesn't exactly support the notion of digg users flocking there:
See my other post: reddit, quite some time, had a blog post about how such analytics seemed to be completely detached from the reality of their internal numbers.
Reddit's google analytics (Jan '11) puts their number of uniques >10x higher than compete and their Jun/Jul number is around 8x larger than compete's.
I guess thats what you for looking at a website that pretends a 1% sample is representative of actual Internet usage for anything beyond painting the most board strokes.
I used to bookmark anything interesting. 3 years ago my bookmarks menu wouldn't fit on the screen at once, and that was just the category folders. Later someone showed me Delicious, and I stated migrating my bookmarks there. But search engines have gotten so good, I don't even bookmark pages anymore.
Why bother tagging when searching's faster? I only have to vaguely remember what the page was about and I'm there (or somewhere better).
Instapaper + Kindle is an awesome combination. Previously I would have a set of bookmarks which were only articles I meant to read later. Now, I just press the Instapaper bookmarklet, and at some time later that day or the next I'll be reading that article on my Kindle... generally during a bathroom break.
I'm sure the author is right to some extent about the rise of Twitter and Facebook being responsible for the decline in social bookmarking, but I think it has a lot to do with the fact that good curation is almost always better than a system that is controlled by upvoters, linkers or tweeters.
Social Bookmarking seems like a meaningless term in this context. Digg and Delicious have only superficially similar characteristics, uses and communities.
It would be like categorizing MySpace and Yahoo Mail together because they both have messaging features.
Not to mention that both sites have individual, specific reasons for losing traffic: Digg had a gradual decline in quality of posts and a disastrous redesign. It was leaked that Delicious was going to be shut down. It would only be news if they didn't lose traffic.
Even ignoring that, the article is just junk. It says very little, and says it poorly. :(