They do have other ways to make money. Brands can be shown as "recommended followers" to users for a price. Brands can also upgrade their accounts to show a banner of somesort (https://twitter.com/nylabone) and support "sticky tweets". Access to their API in large quantities is a source of revenue (I think). Promoted tweets in a user's stream is another source of revenue.

There may be others, but these are the only ones I'm aware of.