Beverage Digest ( Only registered users can see links on this board! Get registered or login!) publishes annual data on flavor, I believe. Of course, it won't be free. Others (e.g. Beverage Marketing) have similar data. Beverage Digest shows a 2003 study here: http://www.beverage-digest.com/editorial/040123b.php . Unfortunately, though it shows a comparison of root beer brands, it doesn't show numbers for root beer as a whole.

A 1999 article states that the root beer flavor segment had been about 2.6% for the previous 6 years, but was growing ( http://www.restaurant.org/rusa/magArticle.cfm?ArticleID=432 ).
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Bottoms up!
Kevin Guske

So heres a tribute toast with root beer in hand to you and the many mugs of suds along your happy trails. --Charles Wysocki, artist and root beer fan, 1928-2002