Clearly you are Google's customer, because they require your custom in order to sell advertising. You are both customer and product.
Because this is about whether or not a term is applicable, let's make sure we have the definitions straight, since I'm not convinced your use of the word "custom" or "customer" is applicable here (though I had never seen "custom" used that way before, so major thanks for prompting me to learn something new!
I went ahead and looked up dozens of definitions for both "custom" and "customer", and every single one of them mentioned some form of money or other valuable goods changing hands. They either said it explicitly (e.g. A customer (sometimes known as a client, buyer, or purchaser) is the recipient of a good, service, product, or idea, obtained from a seller, vendor, or supplier for a monetary or other valuable consideration.), or they said it indirectly (e.g. defining "custom" as "business patronage", which of course refers to money being given, since that's what patronage refers to by definition). But the point is, they all said it.
Which is to say, we're being provided a set of services so that we can be served up as a product (we agree on that), but I would suggest that our receiving those services no more makes us a customer than a cow being fattened is a customer of the slaughterhouse doing the fattening. What makes someone a customer is their payment for the goods or services, but no such payment occurs here, so while we may be the recipient of their services and the user of their services, we are not their customer.