Founded: 2004
First Address: 95 Dunster St., Cambridge, MA
Co-founder Mark Zuckerberg started the company at Harvard's Kirkland House dorm, in the suite he shared with roommates Dustin Moskovitz (now VP for product engineering) and Chris Hughes (once a company spokesman). Facebook's success is already legendary, although Zuckerberg now faces a federal lawsuit alleging that he stole the idea from a rival social-networking site.