I never did that, I did however met this guy who was talking a lot about his girlfriend and then pretended to be her (I caught him about 3 times out of 4...) Why did he do that? That's anyone's guess...

This is my guess - I think that one of the most wonderful things that the internet allows us is to be ourselves, who we are on the inside, but also, to be someone else sometimes - which in a way is just bringing out a part of our personality the way we want to imagine ourselves to be - more beautiful, more daring, more successful, different age or gender and so on.

The next stage in this would be to pretend to be someone we know about like a celebrity, and using that character  to express that part of who we want to be, and so we have Britney spears crazy messed up persona or Brad Pit's success with women and we can, in those strange lab condition, experience how it might feel to be that person.

I just read this psychology book about the different ways people shape their personality it's really interesting and might help you understand this:

http://www.amazon.com/Individuality-Impossible-Project-Carlo-Strenger/dp/1590510151

A word of advice - celebrities are really busy, also, most of them lives a very sheltered lives, a lot of PR people work to not have unintended materials out there on the net. If you meet someone who's a celebrity online about 99% is that they are faking it. 

Also, would you just start following the life of a total stranger \ friend in the way to communicated with those people - maybe the way that you were interested in talking to celebrity can also answer the question of why people were faking it. It's easier to get attention as Angelina Jolie then as Martha Cohen...