Hi,
the Social aspect of search and online promotion is unexplored territory today: I come to this conclusion by the sway of opinion and users from one service to another - hence a word of caution on investing massive amounts of time (=money) into bookmarking efforts.
Take it one step at a time.
How ? First of all I believe you need to carefully evaluate your visibility needs:
- Who are your potential users
- What are their expectations
- what con you provide them
- How does your website (or blog) fit into their search online activities
I'm not one to believe much in Digg traffic - it's quantity not quality.
The move to integrate del.icio.us bookmarks into organic search by YAHOO! will most likely give a boost this bookmarking service
Experiment with other services like Magnolia, reditt and see how what kind of response you get.
Stuumbleupon can generate nice stream of traffic, but then again it very much depends on your line of business - B2C or B2B