It seems like it would be a feat to gather all the user IDs of these famous people in the first place. I'm guessing there's a black market just for that? I used to work on a service used by quite a few famous people, if anybody on the project was unscrupulous, it would have been easy to pass those emails and other personal information on to a hacker.
If you can break into one person's account and get their contacts then you can recurse from there. It's likely that one celebrity knows another and so on.
