Do you really think that it's the threat of the RIAA that has caused file sharing to drop?
First, I'm not sure whether or not that's actually the case. However, even if it is, the RIAA's lawsuits is not, I think, the best explanation. The rise of iTunes, free songs on MySpace, free music videos on YouTube, etc., has made a much bigger difference. People don't do filesharing as much as they used to because they don't need to. Why download a P2P client and expose yourself to risky files of uncertain provenance when you could just listen to Pandora?
I know that my own file sharing activities have significantly dropped, not because of anything the RIAA or MPAA have done, but because there are loads of television shows and movies available to watch anytime, for free, on Hulu.