I agree, but not all of them are social bookmarking sites. As the name of the post suggests, there are plenty of other social NETWORKS too. As the post reads "With over 50 different categories and hundreds of websites listed, you’re sure to find a social networking site that will help you build relevant links and engage with interested people. Is beer your thing? There’s a network for that. What about pets or politics? Religion? Shopping? Smoking? Sport? Spanish? Real estate? Travel? Education? Yep there are social networks for all those too"
I have achieved lots of success, especially for small to medium businesses, by building strong profiles on niche networking sites and providing links from there. Much prefer it to guest posting...