There are a number of things to discuss within this question. Some of these include: Is the social bookmarking website creditable? What are the current traffic levels of the website? What is the process for sign-ups?
In my opinion though, I would be very sceptical in purchasing a social bookmarking website, as the quality of the website, users, and links achieved from this website would be extremely low (in some cases useless). This opinion can be supported by the recent algorithm updates by Google and other search engines, which clearly state that social bookmarking websites are low-quality.
Within having completed the research, my thoughts would be that the email list that you have gained is completely worthless. This is because a number of “black hat” SEO companies currently use fake email accounts to sign-up for social bookmarking websites in order to get an external backlink for their clients; the process is create a fake account, upload a large number of external links/ bookmarks, account gets banned, set-up another fake email account. In all honesty, the email list is probably about 99% fake accounts (unless the social bookmarking website is a creditable one).
Short cuts are not the best technique within SEO. It should be avoided at all costs.