My girlfriend used to work as a graphic designer for an ad agency (several actually, but I digress) and one of her work friends' sole job was to do exactly this for major label clients and film studio clients.
The job was to go to internet message boards, create a profile, pose as a thirteen year old girl, and talk about how great Batman was and how dreamy Christian Bale is, or how awesome the new Vampire Weekend record is.
This guy literally got paid $75k a year to impersonate a teenager online for 40 hours a week. And he was not the only one at that agency, and they were by far not the only agency doing this.
DF