ZECCO, in addition to collecting $$ on margin balances, the stock loan business is very profitable. If they loan your stock out they make money. They also profit from directed order flow. Market makers pay for that and it is normally disclosed somewhere in the fine print. And of course last but not least, the more eyeballs they can prove they have, the more than can get for the ads on the website.
All in all a very profitable business, except maybe for those non dtc eligible security trades.
Signatures are so yesterday!