To answer this question... it all depends on what you are trying to achieve from the social network and the product you are marketing.
For example, if you have a subscription box service and are trying to get your word out there... I would use Facebook. Facebook allows you to achieve quick results and almost everyone in the world is on the platform.
However, if you are trying to market a professional service... I would choose LinkedIn. Even though Facebook has all of your designated buyers, LinkedIn is created to help professionals communicate with each other. So, if you are selling a service to help other professionals I would use LinkedIn to build the business.
Hope this helps.