Anyone who exchanges one thing of value for another can be considered a customer.
No... exchanging one thing of value for another is called being a trading partner; customer/producer means something more specific, and Google/users' relationship is not customer/vendor.
In this case, the user of the service is a supplier of eyeballs, and the only service Google is selling is ad impressions; which are targeted against users utilizing the free service, and Google doesn't need any contract to print ads in the free service.
Nothing of value is actually being exchanged, and there essentially is no contract --- just some policy terms from Google.
Google can shut off a user's access to the free service, anytime they want with zero recourse.
The relationship is not like a customer/producer relationship. In such relationships, the customer actually provides consideration to receive a service and has a RIGHT to receive the service they paid for.
In this case, what Google gets is intangible.