Short answer: he's the founder and CEO of Facebook, he's 26, and he's worth four billion dollars. Long answer: it's a little more complicated.
Facebook currently has almost 500 million members all over the world. Zuckerberg said this week he'd like to double that. Get to a billion.
Zuckerberg famously started Facebook in his Harvard dorm room and he's run the company ever since, growing it to where it is today. What was originally designed as a way for Harvard kids to socialize has now become a regular part of how humans communicate with each other and keep in touch. It's ubiquitous already AND it's growing. So if Facebook is becoming an inescapable part of modern life, and if Facebook is controlled by one guy, we should get to know more about that guy, right?