Could some one pertend to be some famous actor on facebook and even put there picture up and not be them?

They could try, but famous people are verified by Facebook and their accounts have the small blue circle with the white check mark in the center under their profile picture that serves as proof they have been verified by Facebook. This is to help fans know if they are communicating with a fake or legitimate account. Then once fans come across the fake accounts they always get reported to Facebook and get removed.

Of course they could but its Against Facebook TOS Terms of Service to do that tho and if they do that they may get Caught by Facebook and they could get there Accounts Deleted

Happens all the time. If you were famous you wouldn't want to be bothered by riff raff all day long would you?

yes

yes

Yes it is easy to do and there are many fake celebrity profiles on FB.