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...i have read articles saying it could be a big brand push...
How much traffic did you lose, as a percent? Also, was it over just one or two "big" keywords or many rankings of various degrees?
My thoughts on this is that Panda was on the 17-18th & the announcement came a few days late. Nothing significant happened on the 21st (if you look at Mozcast).
Nothing significant happened on the 21st (if you look at Mozcast).