They take a pretty good fee to withdraw money. I'm not sure that would be a decent part of their revenue, though.
For US listed stocks there is no fee throughout the entire transaction (deposit, buy, sell, withdraw): https://brokerage-static.s3.amazonaws.com/assets/robinhood/l...
Robinhood has stated for a while that they have a margin feature they plan to charge for: https://robinhoodapp.zendesk.com/hc/en-us/articles/202853769...