Mark Zuckerberg explains the mission of Facebook in this article from Fast Company:

Five years ago, before Facebook’s IPO, Mark Zuckerberg posted what he called a “founder’s letter” that spelled out the company’s philosophy for prospective investors. “We don’t wake up in the morning with the primary goal of making money,” Zuckerberg wrote. Instead, Facebook “was built to accomplish a social mission—to make the world more open and connected.” Among five specific values that the letter noted (including things like “Move Fast” and “Be Bold”) was this declaration: “We expect everyone at Facebook to focus every day on how to build real value for the world.”