It's long been thought that it is important to get all the content on a web page above the fold. Naturally following that logic was that all advertising had to be above the fold.
This article, called "Demystifying 'Above the Fold'" is important to read for its different approach to this belief. It states that people who's intent is to read content won't notice ads above the fold as they are scrolling down. Better places to put ads are at "social activity" points like comments, ratings, links.
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