Just like FOSS (Free Open Source Software) is killing proprietary software, the internet and amateur porn is affecting commercial pornography sales. From 2005 to 2006, DVD/video sales/rentals dropped from $4.3 billion to $3.6 billion. Amazingly, the article doesn't even mention loss of sales from p2p file sharing.
We warn the reader in advance that the proof presented here depends on a
clever but highly unmotivated trick.
-- Howard Anton, "Elementary Linear Algebra"