In all fiction books, the first or second page has, in small print, "This book is a work of fiction. All events and people mentioned are fake... ect., ect., ect." I found this quote in Go Ask Alice, so is that pretty much giving away that it isn't a real diary? That it is all fake?
Yes, it's entirely fabricated. I don't believe the "This book is a work of fiction..." text was in the original edition, but in one of the more recent editions, they decided to include it. I'm not sure why, but I'd guess that they knew people were catching on to the fact that it's a fabricated diary, and putting it in would avoid legal trouble. Plus, a lot of people don't bother to read stuff like that in books, so it wouldn't be like completely giving it away. (Just my take on it, though. I'm not an expert.)