Depends on what is meant by "small" but even if it was a $2.99 one time fee to be on X then there's a non-trivial amount of KYC info that has been collected, info that goes a long way to building valuable marketing profiles on users. Or skip the marketing angle and just use the saved payment info to develop something like "XPay" -- combine that with influencer campaigns marketing products with a "Buy it now with XPay" option and it could end up being very, very lucrative.