Hi, I run an Adult website and I get a decent amount of visitors from search engine, last month 80% of my website traffic dropped, after a research I found that the most of my keywords (most of them, there still some other keywords indexed) that brings me a lot of traffic are not indexed anymore, after a week my website traffic came back to normal and all of my keywords are back again. Now this month starting last weekend my traffic dropped again to this date, the same thing that happened to me last month happens again. Is there any reason for that ? and why ?
To be honest this is more of a question for the Google forums than the analytics forum. Answering it is to understand the constantly changing nature of Google's algorithm.
If you are seeing this drop in Google Analytics - and if it is significant - go into Google Webmaster Tools and verify your site Google will then often tell you if there is a significant problem with your site.