Ok, so Paranormal Activity 2 did really well ($82 million off a $3 million budget, someone got a lobster dinner) which means they’re going to do a third one and because Hollywood is Hollywood, they already set the release date before the script, director, actors, and everything else required to make a movie. I guess some people work better under a deadline. And you’ll be able to scream in terror at doors opening on October 21, 2011.
As someone genetically disposed to not enjoying horror movies, I've only heard moderately good things about the sequel (which considering most of what I hear about horror movies is that they're complete crap, I guess moderately good is really great) so perhaps a third one will keep the trend alive.
Also, as someone keenly aware of Hollywood as a business, I understand why they would make another one considering the investment they bought on the first two. It's totally cool for me when the studios want to make a buck or two, just as long as they don't turn out shit like a Friedberg and Seltzer. The Paranormal Activity movies look like bastard child of The Godfather and Citizen Kane compared to those pieces of shit. Too harsh? Eh, probably not harsh enough.
Source: RopeOfSilicon