Social media is free right? These platforms need to make money in order to operate. It is like trade you give them your information or data and everything about you, your life and your replashionships in exchange they give you a free platform to keep in touch with everyone you know.
They sell your data to other companies so they can market products and services to you. These companies pay a lot of money to do this, so it is important to keep this in mind and setup your social medias privacy settings to restrict the data