Facebook - why?

Why do companies think it is so important to have a Facebook, Twit or similar account or page when there are millions and millions of people that do not belong to any form of  social media site and have no interest in  belonging to one? 

Why do these same companies ask you to visit their social media media website to "like" them or to enter some competition or simialr which is not available unless you go to site.

I joined Facebook many years ago and was totally amazed at the amount fo drivel posted by even so called intelligent friends plus Facebook monitoring what you look at on the Internet etc.  I gave up on it and have never had any further interest.