I was going to back off because I didn't feel like bickering.
But, yes, I get how you would say it's a prequel. I disagree. For the same reason I disagree that, as guy said, Godfather 2 is a prequel.
The twist is the missing part of the back-story. It continues the story at the end, but otherwise it has a different story that only occasionally intersects with the first. I don't see how that's a prequel at all.
I mentioned Terminator Salvation because by guy's logic that would also be a prequel, and that's stupid.
It's called Paranormal Activity 2. It features mostly new characters, a different story, and then it continues where part 1 left off.
This originally started when the poster was like, it's a prequel, who cares, we already know how it's going to end.
Which, if you've seen the movie, is WAY off the mark. That's what sparked this ridiculous back and forth thing.