Zillow is free to use. It appears that they will make some of their money through advertising. However, they are also soliciting real estate professionals to partner with them in forthcoming programs.
In many states, paying a finders fee is not allowed. For example, here's what Ohio says in an FAQ:
A. No. The payment of so-called “finder's fees” is not allowed. No one may pay a part of the commission or otherwise compensate a non-licensed person who offers information that results in a real estate sale, for such information. Non-licensed persons may not be compensated for performing any acts, including referrals, for which a real estate license is required.
9. Real Estate Licenses. Zillow, Inc. is a licensed real estate broker in various states, but does not broker real estate transactions.