It's not very effective because Google knows anyone could get social bookmarking links. It will help you to get the site indexed but you will need quality links from authority sites.
Its helped many of my sites in the long run, after doing social bookmarking on hundreds of sites every week it generated a great amount of backlinks in a wide variety of websites. It just took a while to see the results.
Partially or marginally helps. Since the "brand update" from Google more emphasis was placed on links and mentions from social sites but IMO you can't just set up accounts and then let them orphan. You have to spend some time to add things to the social sites and only stick to the top 20 or so that matter.
for me, social bookmarking doesn't help a lot when it comes to SERP not like before. But still, it can help as a quality backlink to your site and traffic.
Submitting your site the social bookmarking sites brings more traffic to your site.Building more traffic and more backlinks will help to improve your search engine rankings.