If you pay for a product, you’re a customer. If you don’t, you’re the product. On Facebook, you are the product.
It’s applicable to just about any product or service you do not pay for. From a product management standpoint, every feature introduced is intended to maximize the value of the product. If the relationship is such that you’re paying, then the product is attempting to improve its value to you.
Conversely, if you are the product, then value is being squeezed from you. Incidentally, the advertisers are the consumer in this model. If you think that’s nefarious or shady, try an alternative. And if you find that hard to swallow, remember that the juice presser that’s squeezing you is deliberately very hard to leave.