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Facebook and other Social Media Platforms will be more effective with respect to ROI.
It is depends on what are you looking for in return? What is your target market and etc? If you want create brand awareness then FB is a good place. But if you really want sell something radio is much better (looking at what radio are you intrested at) this question will get different answer at different situation.
better ROI depends on your plan and your action plan so keep an eye on your channels and it will give you the best ROI, but i can not tell you now what is the best