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There is no magic number of frequency of links that will secure you good, long-term high rankings. Richard is right, you shouldn't consider link building as a painting-by-numbers game where you decide on a number of links from one type of source and fill these numbers into a chart as the links are built. Check out the link building category on the Moz blog. It contains years worth of high-quality content about creative, sustainable link development ideas. Please keep in mind that "social bookmarking" is a very outdated technique. Depending on what you actually plan on doing, a lot of these links may be nofollowed as well, making their usefulness for SEO purposes negligible.