The site's main purpose (in case you're just coming into the light after a few years on a desert island), is to sell advertising. Its offshoot purpose is to connect people, of all stripes, for free. But it's those connections which sell the advertising.
The big three older social media sites, when you strip away the celebrities and the companies, can be a boatload of errands or a standard-form holiday letter. You know the kind, where you're told little Suzie has taken up the clarinet. Over and over ad infinitum.