The main purpose of social bookmarking is tagging your site with specific keywords. Social bookmarking does not guarantee it will give you traffic, however, do-follow social bookmarking sites can increase your PR.
If you want to increase the traffic of your blog, social bookmarking is one of the best ways. However, revenue is not guaranteed all the time.
Social bookmarking is primarily designed for traffic, but you can still build backlinks from it. I bookmark my sites manually. I don't recommend any automated tools for that. They are crappy and often used for spamming.
Social bookmarking is for....... It seems there is a difference of opinion but in any event. How does google like the low pr value of the page these back links are coming from? It would seem to me that any social bookmark back link is coming from a page that has a 0 page rank. I know a back link is a back link so you need lots- and I mean lots! Right!
They are good for traffic and links. However most people argue that the traffic comes through isn't retained hence they are only good for links.
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Yes, I think it is the main benefit, a backlink, and the other is to get other backlinks if your site catch the interest of many members on those sites.
here is a list of bookmarking sites which would be of help to increase traffic: http://totalaffiliates.com/25-bookmarking-sites-to-get-traffic.html