Adsense Revenue way way down

I've been using Google's Adsense service to deliver ads to my visitors for about a year now. Since January of 2005, my revenues have dropped dramatically. I was earning an average of $100usd a month until January... Now, I'm at $25 to $30 a month.

This is spite of adding two new sites to my lineup.. Purehope.com and Thirdsite.com. My original site, Plagiarized.com still generates the bulk of my revenues.

I'm mystified, and in no way loyal to Google. Maybe it's time to switch to Overture... I wonder what MSN-Search's ad delivery revenues will be like.

My ad-revenue plan is to create more and more very targeted websites. X-box video games, Canadian movie reviews, job search tips, parenting, etc. I think the more targeted your content, the higher your potential revenue from ads.