The process started crudely. You would just take the list of accounts that had joined a fan page for some show or band, and assume they were fans.
Then came the first tentative steps into analysis. Finding causalities and likely crossovers.
It only got more and more sophisticated, as soon every post, every approval, every single interaction online was used to create a fuller picture of who was on the other end of the account.
Pretty soon, you could predict what they would do.
Then you could pre-empt what they’d do.
At that point, did you really need them anymore?