They don't pay only for using it. They pay to market it to their millions of followers as the best thing the world has seen since the internet.
The network/fanbase is usually gonna try anything they suggest, especially if it's free to signup.

They could even go so far to say that if you power up x amount of steem (500 steem e.g.) you will get a big upvote from me, your idol...
Some crazy people would pay thousands of dollars to get recognizes(upvoted on social media) by their idol.