Social media platforms rely heavily on centralized data storage and opaque data-sharing practices, exposing users to privacy breaches and unauthorized profiling. This paper presents a blockchain-based privacy control system that enables social media users to define, monitor, and revoke consent for data access and advertisement targeting. The system employs smart contracts deployed on the Ethereum blockchain to store user consent, manage third-party authorization, and maintain immutable access logs. A Design Science Research and Object-Oriented Analysis and Design approach guide the development of the system architecture, including user interface, smart contract layer, blockchain network, and audit dashboard. The implemented prototype is evaluated using gas usage and execution time for core operations set Consent, set Authorization, and request Access. Results show that all operations complete within milliseconds with moderate gas consumption, indicating that on-chain consent management is feasible for interactive social media scenarios. The findings suggest that blockchain can enhance transparency, user autonomy, and accountability in social media privacy management, although scalability and regulatory integration remain important directions for future work.