My site's google visitor amount turned the normal amount (it was 1/10 since 28th apr) about 5-6 hours ago. My main key phrases come back to the 1st page from the 4th and 5th pages. I am happy about that I wish luck to the other who have been suffering from the sudden drop of google visitors. I think that sudden serp drop was because of that pr update. Now that pr update settled down completely and google serp flactuations turned to normal.