Did your rankings of these sites also drop? Am I correct in assuming that all of the sites are now back up to normal visitor levels?
When something like that happens I usually check my social bookmarking accounts like Digg and Reddit. It's easy to get banned on those sites and if you don't have enough accounts to spread out your risk, it could seriously impact your ranking. I've had 5 months worth of work completely wiped out on Digg before I started to manage multiple accounts at once.
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thanks to everyone who tried to help me. There was no reason to worry, all three site are back to their old visitor levels. As I said this happens about once every six months, normally it lasts a few days, now it lasted a week and a half, that is the only difference. I did not change anything on these three sites during this period.