social sites are useful in terms of traffic, but in conversion maybe it's not. But some social sites are really useful like Digg...
It's very useful for me. My site indexed in few hours in google after write a post in my blog. Importance of submitting posts in digg Use BlueDot :: Good social bookmarking system
if there is not nofollow on their links , social bookmark sites will help your seo but most of them are using that tag in their links. i think social bookmark sites are good for traffic
Digg and Slashdot are the top 2 social networking sites. If you get "dugg" you can get some very significant traffic!
They are useful because they give you traffic. the other thing some of them gives backlinks to your site.
Social networking sites like facebook, twitter, tumblr, myspace, etc. is a good way of promoting or making your site visible enough for the viewers. These social networking sites will help you to increase your traffic but not in backlinks. Just make a interesting post or topic to drive viewers to see more of your post.
we can share our feelings, findings, knowledge, photos, videos, etc. And also we can creat a good relationship with an unknown person.
If you are publish your news with bunch of site with same class c. There is no use of it. IMO Social bookmarking is used for indexing purpose only, if your content is catchy, it become a traffic.