Fake.
How many of you actually knew it was fake when you read the book or watched the movie? I don't think it's right that it is being advertised as a true life diary. I've read fictional diaries before about drugs and what not, so why couldn't they've just written that across the cover. I don't think the book would be as popular, but I'd still like to know .... maybe just put it in the epilogue or something.
Its better to be fail in originality, then to succeed in imitation.