ZydoSEO - 5:17 pm on Feb 14, 2013 (gmt 0)

Some things to consider when trying to pinpoint a root cause of such a drop:

Were there any messages in Webmaster Tools from Google?

Google has gotten much more transparent than they were in the past. They are frequently notifying webmasters via WMT when a site has issues that might lead to a filter or penalty. In the past there was zero communication between webmasters and Google. While it's still not ideal, something is better than nothing these days.

Did your traffic suddenly drop 50% on a given date? or did it gradually drop to 90%, 80%, 70%, 60%, 50% over weeks or months?

If it dropped on a given date then that is a BIG clue. Look to see if there were any major updates at Google on that specific date (Panda? Penguin? EMD? Above the Fold, etc.)... (Mods note: Take a look at our Hot Topics [webmasterworld.com] section, pinned to the top of the Google SEO forum home page, which will lead you to our various breakdowns of the Google Update History [webmasterworld.com].)

Did you lose traffic for a very small set of keywords (1, 2, 3, for example)? Or did you seem to lose traffic for most of the keyword phrases for which you were previously ranking?

Did you lose traffic to one URL? Or did you seem to lose traffic site-wide for all of your URLs?

What changes were made to the site on the weeks leading up to the drop? Navigational changes and/or changes to the link text used in your internal linking structure? Design changes? Were URLs changed/deleted (and did you 301 redirect them if so)? Was content changed, added, or deleted? etc.

What type of link building was done in the weeks leading up to that drop? What link text was used in the links? Which URLs were being promoted? Do the URLs previously promoted with those associated link text coincide with the URLs and keyword phrases for which you have lost traffic?

These are the types of questions I try to answer when tracking down such drops in traffic. Hopefully, you have a web analytics package installed (prior to the drop).
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[edited by: Robert_Charlton at 6:20 pm (utc) on Feb 14, 2013]
[edit reason] add reference to Google Update History [/edit]