Now, that's not MySpace or YouTube growth, but it's pretty solid, particularly considering the fact that Yahoo has not done a lot to promote the social bookmarking application. Most of its growth is from viral marketing.
One question I would have is how active these million users are. Are these people who have tagged 5-10 pages or 100? Have these users all bookmarked new pages in the past 30 days? 60 or 90?
My second question is how many of these users are using it truly as a social bookmarking tool, i.e. sharing their bookmarks and adding others to their network (and looking at their bookmarks)?
I'm a delicious user, but I use it more as a portable bookmarking tool so that I can access my bookmarks from my desktop or laptop (or last year, from a PC on a cruise ship). I rarely take advantage of the social capabilities of delicious.
This is why I think the rumored Yahoo acquisition of Facebook would be a big boon to the company. Yahoo has some great social networking components in Flickr and Delicious, but they remain somewhat niche products. While I commend Yahoo for letting its acquired companies remain independently managed (I've been involved in too many acquisitions where integration was a nightmare), having a strong platform like Facebook, with tight integration of these other social networking tools, could drive usage.