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Radically changing rankings are normal for a new site, or a site with little authority. Established sites with good off-page optimisation rank more steadily, holding the same or similar rankings week after week. We would need to see your site to decide if you had an over-optimisation problem, but I find that unlikely - the ranking issues you describe sound more like the site lacks strength. Generally, social media sharing, bookmarking and forum promotion are not enough to make a big difference, especially in more competitive markets.