I dated a woman that had this job, but using Facebook (this was some years ago too).
It was not just marketing, but PR too, for example she bragged about when Whirlpool had a serious problem with one of their washing machines in my country, and it was going viral, so Whirlpool paid the marketing agency to make all fake profiles (even ones not belonging to their campaigns) make other viral posts to drown out the washing machine one. She bragged about two things: 1. It worked, the "counter-viral" campaign successfully made people ignore the washing machine issue. 2. They got paid ludicrous amounts of money (she hinted that each account with 5000 followers, that was Facebook limit, was worth some thousands USD, and that they had many, many accounts, precisely to go around that 5000 follower limit, for example if a company ordered 50.000 followers from them, they would setup 10 fake accounts and try to attract 5000 real people in each, each account with different hobbies to try to attract different people).
EDIT: some other interesting info:
* They had a huge cache of personal photos, to make it look like the person was real, there are even stock photo agencies that create these photos, for example take a female and a male actor to tourist destinations and take pictures to make them look like a couple doing romantic things, then they would release the photos slowly, so people wouldn't notice it was fake, and to stretch their money for photo purchases.
* They would get sometimes employees with real hobbies, and have them take care of fake accounts with same hobbies, so a surfer for example would have some 8 accounts under her care, each for a different company, and write some surfing content that is real, but tweak and then repost on all 8, with differences, and adjusting to fit each profile.
* Another technique at the time was the sale of "instantaneous company profile", you made a fake profile, and made it get genuinely popular with a target demographic, then you sold it to someone, after the sale you could edit the account to turn it from a person to a fan-page, with the category correct for the company that bought it, so a company could buy a fan-page for them with 5000 real followers that actually are about the subject. I never asked how often people would realize what happened and unfollow... but I guessed most people wouldn't bother with unfollowing anyway.