I think Web directories takes too long time to process the result of the back links, And social bookmarking will helps to drive the traffic, Back links and PR ranks.

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Both techniques are good but you have to choose only authority directory site like Dmoz and Authority Bookmarking site like Diggo, Reddit & StumbleUpon.

Both are necessary but my point of view social bookmarking is good for generate traffic.

I think both are important for link building.Directories submission and social bookmarking. Both are helps you to increase website traffic.

Both are beneficial. Submit your site's link to high PR sites. Social Bookmarking is more active.

They both have their advantages and disadvantages. For getting maximum revert of your efforts, you have to apply all these activities. It will be help you more if you are getting these back links from Do Follow links.

Both, you should be backlinking from directories and advertising your website via Social media, as long as you have great content and you can offer your cliental great deals and offers you wont fail.

I may sound like a broken record but "Content is King"