I love this question -- it is about adoption of a new solution. It is about the barriers to adopting a new solution from the perspective of the customer experience.
I don't use Delicious. But I am very excited to tell you the reasons why I am not in the active market to switch from the book-marking program I use. I think that they are general adoption points that are immediately transferable -- and part of the challenge for any start-up in this type of space.
Visibility: I like to use a social networking program that other people are suing so that I can maximize the benefit of the social aspect of the program. I like the simplicity of it's logo/icon on articles I read and that I can simply click
Business Stability: I like to store personal information that I would like to use with a company that I know will be around in 6 months, and 6 years. I don't really like to have to change my notes, my password repository, my recepies every time the platform I chose decided to close down, discontinue to sell to someone else. Call me lazy -- but it seems like I have better things to do with my time.
Expandability: I want to know the platform I chose will get better with more features and functions. To this end I prefer ones that have established such a track record.
Integration with my other solutions: Bookmarking is part of note-taking and task management for me. My bookmark solution needs to integrate with the solutions I have chosen in these areas. The social aspect of bookmarketing needs to integrate with the social platforms that I have chosen. If they don't I am creating a new island-- and I am not in the terra-forming business.
No pain: I don't have pain. Change causes pain. If I don't feel pain I am not likely to do something (change social bookmarketing platforms) which has a likelihood of causing me pain. Investing time in learning a new platform and new features and figuring it all out-- that causes me time away from income generating activity and that causes me pain. Until my solution can't do something I need it to do -- well, I am not even actively looking for something else.
An object in motion tends to stay in
motion, and object at rest tends to
stay at rest.
I apologize for the potentially depressing nature of this post to your exciting adventure in launching a new social bookmarking site. I am a fairly early adopter. Inn some areas I am bleeding edge. In others I am cutting edge -- and with others (like Kitchen Utensils and canoes) I am definitely old school. I may or may not be part of your target audience.
From a B2B marketing perspective the above 5 can be generally re-written for almost any new product/service entering a well established market. Doesn't mean that there are not ways to break in -- but that is what a good marketing professional will do and can do.