Here’s an interesting exercise. Take the total number of news stories, social media mentions and tweets about Apple (AAPL) over the course of, say, three months. Rate each hit for its impact and value — positive, neutral or negative. Calculate what it would cost to generate that kind of buzz if you paid a public relations firm to do it.

That’s what a Long Island, New York-based company called General Sentiment has done in a report issued Monday. By its measure, Apple is the No. 1 brand leader for the third quarter in a row, having generated a total of $4.5 billion worth of what the company calls “media value” in 2010.

But it also reports that the value of Apple’s buzz fell dramatically last quarter, from more than $1.4 billion to $941 million and change.

Google (GOOG) is also down, but not as sharply. Microsoft (MSFT) and Yahoo (YHOO) are up. Go figure.

For more about General Sentiments’ report and methodology, click here.

[Follow Philip Elmer-DeWitt on Twitter @philiped]