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Yes, having social bookmarking as your only strategy will not yield great results (probably just waste your time) but doing it as part of a larger promotional strategy helps you get a boost in rankings + is viewed by member of those bookmark sites. You get found by more people quicker which helps the promotion > engagement > rankings cycle start. (e.g. if you rank page 1 for one of your services, you are more likely to be found and link to/talked about.