Social bookmarking seems to be the way of the future too.
I don't think directories are a significant part for SEO.I will definetely recommend social bookmarking with my hands up.Actually I don't use directories for maybe 3 years already.And I think lots of people share my opinion.The pr5 and PR6 directories will MAYBE post your link in 6 months .Why is that?I submit a link in reddit which is PR8 for less than a minute.
I think it would be wise to use both of it. Diversification is the key if you wanted to achieve long term success.
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i think social bookmark but its depend on the domain authority
If you are concerned about generating backlinks through this question than no doubt Directory submission while if you want traffic than Social Bookmarking
Why you dont doing both ???
I think social bookmarking is a great tool in SEO as you get instant links. But all social bookmarking sites are not helpful.
You should use only the selected bookmarking sites.
Both directory and Social book marking are good.Its will give good back links.
It depends on the site. Some social bookmarking sites are quality and some are spam. Same applies to directories.
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Both of them are effective as long as you submit your website URL in high PR sites.