They all used to work, but things have changed. I can get 10,000+ retweets, and they were a quick way to rank at the top of the serps, but the problem is Google changed the algo to prevent this. Check the studies on other SEO forums, plenty of people testing these out and they don't work as good as they claim to.. People were experimenting with this for a long long time now, and it's not that it's ever been ignored, it's that people found out it's not as effective as people "claim".
But.. Social bookmarking can boost you into the SERPS as a small part of some SEO. There's hundreds of these sites you can drop your link into and then run a link pyramid with... So I'm all for social bookmarking, just not +1's, and retweets, and likes, etc..
