Despite the fact that both get released in compiled volumes costing somewhere between 400 and 1000 yen a few times a year, the market for manga differs in many ways from the market for light novels. For one thing, it’s a much smaller scale market; the most popular light novels sell hundreds of thousands of copies immediately after their release, rather the millions that One Piece/Fairy Tail/Attack on Titan bring in. Too, light novel adaptations tend to succeed much more as a function of their initial popularity than manga does. To get an in-depth look at how anime adaptations of light novels have impacted the source material, I’ve plotted the sales for their first two weeks of solicitation over time, with the airdate of the anime superimposed. See this doc for the raw data, and compare the 2011data for manga adaptations.
Note: I plot the two-week totals, rather than the one-week total, because even the average the first week total is vulnerable to a series that comes out on the last of the Oricon tracking period and having its sales numbers hamstrung. Kore wa Zombie Desu Ka had 4 volumes in a row come out on the last day of the week, obscuring a very real anime-fueled boost in sales.
Based on what I’ve seen of reactions to Free on the internet, it seems like a large quantity of people are ruling it out with one glance at the promo material rather than 20 minutes of episode time. It’s becoming increasingly obvious how much of a shame that is, because this show is complete in ways it didn’t even have to be to be an enjoyable ride.