Digg.com - Number 1 social bookmarking site?

It seems that digg.com is the most popular social bookmarking site. Why is this so? Its supposed to be a news voting site? But people are using it for bookmarking?

http://www.ebizmba.com/articles/social-bookmarking.html - has reported digg.com as number 1 social bookmarking site.

del.icio.us which has more proper social bookmarking features is listed at number 4. interesting.

So are there any other websites that is not meant to be what its meant to be in a certain category but ended up being the most popular in that category?