ICv2 notes that the sales drop was heavier in bookstores than it was in the direct market, and cites the loss of big-name anime on TV as a factor in the drop. This tells me that the audience that's being lost is NOT so much the hardcore fan-- though surely the regular consumption of scanlations isn't helping --but rather the casual fans, the people who wouldn't be on this site in the first place, most likely.
The rest of the factors listed are ones we're pretty used to: manga fans aren't "collectors" like comics fans, series are long and hard to collect in their entirety, fewer titles are being released, etc.
Also of note is that manga has seen a similar pattern in European markets-- a large boom followed by a drop before stabilizing --so in all likelihood we can hope for the same here. But that means you guys'll have to keep on buying your favorite series! What's on your shopping list these days?
Is the 20% referencing 2008 sales or the same year that the drop in 2008 is referencing. If it is the former than saying they've dropped by over a third is incorrect.
I do collect the manga though, and haven't sold off anything so far; though it is starting to take a bit of space.
I should note that if it wasn't for my credit card and the joys of the internet, I'd be pirating everything, since few stores carry this stuff, even though I live in a city of one million people. I can understand why a lot of younger fans are so into piracy, since it's really hard to get a hold of things unless you live in a teeming metropolis.
I think the recent economic down turn was the basis of this decline. Alot of things have sufferd in the us due to it and Manga would be no differant.
Like, given the number of downsizing and companies going under, it wouldn't surprise me.