There is no reason to use social sites just to get backlinks, that kind of backlinks will not help you rank higher, just use the main social networks, facebook, twitter, google+, pinterest, linkedin and will get a lot of traffic, (not by spamming people but by networking and building relationships) additionally these social networks help much better your rankings than some unknown theoratically high page rank social bookmarking sites...

search in google. you will find 1000 bookmarking sites.

Thanks to all for the suggestions.