Push notifications around a pre-defined 'watch list'? Notifications of major stock movements? Showing 'popularity' of a stock? 'Most Popular Under $25'? Gifting people free stock for referrals?
If I had browser data from everyone on RobinHood, and was able to track which stocks they were looking at, I might be able to make money front-running.
If all RobinHood users followed sound "buy diversified ETF and hold" investing advice, my understanding is that they'd go bankrupt.