According that chart you can see the visitors behavior. Like you can generate the custom report that on which keywords the visitors are came. They might be not land on relevant page. I know this is not the perfect solution but you can start the initiative. At-least you will able to see the flow which is suitable or not and according to that you can do some surgery for your webpages.
Since you have said that the site was not down during the period of low visitors I fail to see how it could have to do with your hosting company. Granted the fact that three sites with the same host dropped at the same time would make you wonder, but either your site is viewable or its not. I would sign up with one of the free site monitoring companies and have them monitor your site on an hourly basis just to be sure.
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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Hi,
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.