One of my sites has been dropped-off completely for long tail keywords for weeks now. According to what I've noticed site was receiving traffic for long tail keywords. It is really hard to assume that this caused due to recent panda update which was launched on 21 December because the drop was emerged since 18th od Dec. No major changes were done other than some url structural changes it its existing url structure. By looking at the site and nothing jumps out at me in terms of on-page factors.
[edited by: goodroi at 11:57 am (utc) on Jan 15, 2013]
If you are using internal tracking you should see that the traffic is still on your website but you just do not know the keyword it came from. Google from the 10th-19th have forced users of firefox and chrome to use https, this means they will be in the (not provided) bracket (ie if you search in the google browser bar). I thought i also lost traffic but overall site traffic is steady.