There are dozens of social bookmarking services. If you tried to promote your site on all of them, you would probably end up spending more time on submitting pages to those sites than you would on producing new content.
Most of those sites are very sensitive to ‘spam’, and are hostile towards people who are clearly shilling or promoting their own web sites. So, if you have a strong community already, why not encourage your users to promote your site for you, rather than doing all the submission work yourself.
One effective way to do this is to place social bookmarking links at the bottom of each article. These links take the user to the submission page of their favourite social bookmarking site, so all they have to do is enter a quick description, and submit.
One such script is available for download here.
This script was originally designed by Hugo Haas. I have been using it on my web sites for quite some time, simply updating the social bookmarking links as necessary.