I think it is always better to do social bookmarking manually. But if you want to make use of some tool, you can try Onlywire. It can bookmark your site once in over 20 bookmarking sites.
I m not much in Social bookmarking ... If i got the fastest way to do social bookmarking than i'll definitely try ..
The fastest way is to use automatic submission, but you won't reach any of the most popular bookmarking sites like digg, reddit, stumbleupon etc (even if you succeed to submit a few bookmarks to them, you'll get banned soon). The best way is to use semi-automatic tools like www.socialposter.com or www.addtosocial.com. This is still manual bookmarking (the submit form is submitted manually by you), but it is auto-completed, so you have to enter the data (url, title, description, tags) only once for about 100 sites.
Hey, if you go through manually then it's the best way for good result but if you try with software then may be it will be considered as SPAM
Without a shadow of a doubt, the 'Fastest Way' would be to use an automated tool. Some have been mentioned above. If you are using a tool make sure that you complete your account profiles with unique information (& photos where required) & don't just bookmark your own sites over & over again. Identify web properties in the same niche (that are not your direct competitors) & also bookmark them from your same account. This will add to the relevancy of your bookmarks & increase their buoyancy in the SERPS.
i manually do social bookmarking on different popular sites like twitter, stumble upon, Digg, Reddit, Delicius, mixx etc where i can achieve quality back links and traffic to my website.
I also submit to social bookmarking sites manually, better safe than sorry. It's hard to risk your site's PR on using softwares because Google can detect that.
PHP Dir Submit is wonderful tool that I am using for directory submission, article submission and bookmarking. Its a semi automatic tool. http://demo.phpdirsubmit.com
Bookmarking demon is none other than social bookmarking software which help build backlinks to your websites. Backlinks are the number one factor when it come to ranking in the search engine, and everyone know search engine are the best way to get free targeted traffic to your website.