There are many ways to earn money from Facebook.
1. You can have your own page created on Facebook. You can start posting content like discussions on a various interesting topic, videos made by you and images you captured. Visitors come and see the content. If they like the page they keep visiting and if the topics are good, more visitors come. Once they cross a particular mark, you can ask for paying money to you from Facebook. You will get cash based on the number of visitors.
2. On Facebook, there is a page called the Facebook market place. You can earn money by placing your spare goods on this page. Buyers will discuss and bargain with you the price. You should keep the minimum price at what you want to sell in your mind.
There are many more methods for this activity on Facebook.