It really depends on your business - is it B2B or B2C? And also what kind of visitors do you want to your website.
If it's B2B then you should start with LinkedIn as you'll find more formal & professional audience engaging in industry or market related groups discussions that you can participate in. LinkedIn can also provide many hot leads with good purchase intent. You can use Facebook as a secondary channel for content marketing and nurturing leads who are relevant to your business but not ready to buy your product or service just yet.
If it's B2C, go for Facebook, don't use LinkedIn. However, in this case, you can also try other networks like Pinterest, Instagram, etc. I would suggest try all major social networks for B2C websites before you decide, cause you never know which one would click for your specific business.