I think social bookmarking tool is useful today, mainly for reputation management.
Because, you would be able to push the ranking down of your website is posted as complain on other websites/forums..
The thing is you need to use low quality social bookmarking sites, all of theme should be on one platform, use same title, description to all the social bookmarking sites and you would be able to take ranking down of the page where your site is appearing as complain.