I've seen a lot of self-diagnosed autism, particularly online.
Autism can be more extreme than people being socially awkward. I worked at the local mall for the past year and gave out free samples occasionally, so I ran into a lot of people. Two children in particular stand out significantly to me. When one boy and his father approached me when I was giving out free samples (candy offered in a tray, I had a 'spoon' so I picked up a piece and placed it in their hand) I was about to offer him the piece when he grabbed my hand and was pulling the spoon to his mouth. His dad pulled him back before he could bite it or anything and explained that he was autistic.
The second child I experienced while running the cash register, and he found an Alvin and the Chipmunks doll we were selling and started screaming and physically harming himself when his mother (maybe just a caretaker) told him he couldn't have it. He actually started throwing the doll at me and my co-worker for us to ring it up and his mom had to literally drag him out of the store as he screamed "Alvin!" and slapped himself.
Those were the only extreme cases that I've had any experience with. Now, I've known people who will diagnose themselves as autistic for whatever reason. I've seen one girl in particular do this as a scapegoat for having a crappy demeanor and pretty much just being a complete jerk to everyone she meets. She could have legitimately had a mild case of autism, but based on my own experiences with her (her saying she's autistic one day, the next day saying that she has no form of autistim at all...?) I think it's safe to say that she's the type who uses these disorders in her favor, so to speak.
I believe that autism is a very real and very, very misunderstood disorder but because of its nature people will often diagnose themselves or be misdiagnosed by doctors, even, and it all just fuels the misconception on the disorder. It's difficult to wrap one's mind around how autism works because there are so many levels of severity in it, one boy at my college has a mild case where the only thing that makes him stand out is that he approaches everyone in the student center to avidly discuss My Little Pony and can't pick up on obvious and blatant clues when someone doesn't want to talk. He's in a completely different zone than the boy who harmed himself and screamed because of a doll.
tl;dr diagnosing autism is a pretty fluid practice because of the levels of severity, and this leads to a lot of misconceptions about people with the disorder.