For big companies that benefit from being able to say they have lots of users there is a big incentive not to be good at finding fraudulent accounts. I worked in Big Data Analytics at one of these companies $10B+. We were separate from the fraud department. They'd filter out the fraud accounts and we'd have to re-filter because their behavior was so out of whack it would mess up our analytics. We tried to move our filters upstreams and teach the fraud dept how to identify these accounts but absolutely no-one was interested. Also it's common to have bonuses tied to user numbers.