Retention and advocacy are important parts of the buying cycle. Where search engages, social keeps consumers attached to the brand.
That’s all you need to understand about Social Media vs. Search Engine Marketing. Search is about selling to new customers. Social is about follow-on sales. Define which it is that your business profits from and you can then allocate resources accordingly. If you sell kitchen appliances (which have a purchase cycle of 20+ years) then Social Media is probably not your focus, whereas Search should be. On the other hand, if you run a hair salon (which must have repeat business to thrive), Social may be the dominant channel for you.