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Unless you're mega-popular (like Amazon, Wal-Mart, etc), I would expect things like this from time to time. I tend to find that precipitous drops are due to a shift in algorithms. You might be going "well duh", but it's important to note that an algorithm change can cause shifts like this on a temporary basis while either the indexation process catches up or Google evaluates a set of changes (internally or by using quality evaluators). My bet is it's the latter since Google announced they rolled out a lot of new changes (which is not something Google does on a regular basis). That, to me, screams "testing" (remember that Panda did the same thing to a lot of sites about 1 year ago). Eventually the SERPs return to "normal". That having been said, are you sitting still? If so, that's not going to help you in the long run. Add links and content. Keep up with your competitors. Remember, they want to sell your products as much as you do.