Facebook is a (tool) used for a myriad of purposes based upon the person or company using it. Most people use it as a "one-stop" place to stay in contact with friends and family to update them with what is going on in their life. The goal is to "connect" with others.
Businesses, authors, and entertainers use it to promote or market themselves. This why so many businesses say "Like us on Facebook". Each time you "Like" something it appears on your wall for all of your "friends" to see. Social media marketing is big!
Some people have used it to find dates. Others have used it to bully people or spread rumors. A lot of people use it to (share) articles, jokes, Youtube videos, and other things they come across. Facebook is whatever the (user) decides to make of it.