I'm not really sure they are even immitating those people, they are just using their picture.
I, myself have done it once, and only once. I created a seperate Myspace profile, because my current ex was cheating on me, and I used a picture of a Barbie, and a Penn name *that is my personal Penn name* and friend requested her.
Between conversations and her stupid little surveys I was able to find out that she was telling all her friends that she was dating *and going to marry* the guy that I was with and planning a wedding with.
He never knew that I knew this. I did confront him by telling him that one of her friend called me to tell me this was going on. Shortly after, he dumped me. Shortly after that, he married her. In the process, he swears that she wasn't saying any of that *even after I fessed up to the truth and that I had seen it with my own eyes* and he never cheated on me. THEN he admitted *finally* that she did say it, but she was lying. When I asked him why he would marry a liar, he said it was because he knows he can trust her. 
What it really came down to was this, he parents hated me (I was a whore for kissing a boy behind his back in high school) and she was the sweetest girl ever (even though she screwed idk how many guys behind his back while he was at sea *yes, it's the guy from the Navy I used to date*). Still think that I was so horrible and she was so great all has to do with the fact that her family is millionaires while my family just scrapes by.
I f***ing hate money 