This is normally used for e.g. selling fake likes. It's taken as a given that you will have regular churn as accounts are detected, occasionally high spikes of turnover as fraud detection and safeguards change, that kind of stuff. Often the action of converting network behavior into currency activity is what burns accounts.

Given an alternative way of rapidly converting part of that network into a compelling quantity of USD, and the fact that you're routinely rotating accounts anyway...offering to pay money or money-equivalents for Facebook connections in an automated fashion is pretty much screaming for fraudsters to call in the biggest airstrike they can before you come to your senses.