Every time you add a bookmark to one of your pages in one of these   sites you create a backlink. If you bookmark every piece of content you   create in all of these sites you can build a lot backlinks over   time. For best results, you want to make sure that the   bookmarking sites provide a real, do-follow link. There   is much debate as to whether or not no-follow links are still useful so   I won’t get into that here. However one thing you do want is a real   link. Several sites don’t provide real backlinks – they use their own   form of dynamic link which does not count as a backlink.
One other thing to be aware of is that even though these are your   personal bookmarks, you need to ensure that they are public   – that way other people (Google really) can find them. But be careful,   some bookmarking sites such as Connotea  have a specific niche and probably wont appreciate your toenail   clipping bookmarks.