I too agree that social bookmarning is not a waste of time. It also help in serp improvement. IT also help in new content crawl fast by Google & others.Bookmarking is not at all a waste of time.. If you have the ability to do it in a proper way, you can see a lot of improvement in traffic and as well as keyword rankings. If you are following the same old style not implementing something new, then I can say that you are surely wasting your time.