The idea of it is actually quite terrifying *Major Spoilers*
So this movie as a whole is okay. Not great, not terrible. My thought the entire movie was how utterly terrifying the idea is. Think about it:
It's just a regular day hanging out with your group of friends doing whatever you normally do as friends (party, go to church group, sit around the house and smoke pot, etc.) Suddenly, you bring up one of your friends' names and all the others have no idea who you're talking about. Now me, at first I would think it's a joke, nag my friends to fess up to it, and get a little pissy when they don't but let it go....but then it keeps happening. All of a sudden your group of friends is whittled down to a much smaller number and you seem to be the only one that remembers. Both conclusions I would come to freak me out: Either I'm *beep* crazy (which in reality would suck!), or, the creepy supernatural stuff is actually happening, your friends are dying, everyone now thinks you're crazy but you're not, and you have no idea how to stop the rest of your friends (perhaps including your boyfriend/girlfriend or sibling) from buying the farm. I think the latter sucks worse.
Just my two sense.