Mark Zuckerberg left Harvard University in 2004 to pursue his big idea. Seven years and 800 million users later, the 27-year-old founder of Facebook returned to alma mater Monday for the first time since he dropped out to announce he's hiring.

There's a lot of really smart people here and a lot of them are making decisions about where they're going to work, Zuckerberg said to a crowd of several hundred students and reporters standing outside the university library. We're just getting started. The next five or 10 years are going to be all about the different products and industries that can be rethought.

The crowd of Harvard students who turned out to get a glimpse at the university's most famous alumnus was so big that campus officials needed to set up temporary barriers around Zuckerberg.

There are relatively few tech rock stars whose names are known by people all over the world, said David Malan, a Harvard computer science professor who also cited Microsoft's Bill Gates and Apple's Steve Jobs as two other such examples. [Zuckerberg] really is in that category.

Bill Gates, not so much, he's too old. Steve Jobs is up there. But Zuckerberg is the biggest, said Joseph Botros, a freshman computer science major at Harvard.

Before he returned to Harvard, most students only knew about Zuckerberg from what they heard, read about, or saw in David Fincher's 2010 film The Social Network, which depicted Zuckerberg as a conniving, untrustworthy misfit only out for his own gain. By seeing and listening to him in person, many students formed a new perspective about one of the world's youngest billionaires.

I feared that Mark Zuckerberg would be very pretentious, said Lucas Freitas, a freshman computer science and economics major, and Botros' roommate. It turned out I was completely wrong: Mark Zuckerberg is extremely passionate about his project and is actually a very cool guy.

Zuckerberg also made a 75-minute presentation to students only, in which the press was barred from attending. Students came away very impressed and encouraged.

He gave a very motivating talk, Freitas said.

Zuckerberg offered some excellent advice on how to become a great engineer, but Zuckerberg's story was enough inspiration to students by itself. Within a year of leaving Harvard for Palo Alto, Calif., Zuckerberg's Facebook had grown to one million users and had attracted investors including Greylock Partners, Accel Partners and PayPal co-founder Peter Thiel. Now, his site is the world's most dominant social network with 800 million global users and 3,000 employees, and Zuckerberg has claimed several titles of his own, including the world's youngest billionaire in 2008 and TIME's Person of the Year in 2010.

It's an empowering story, especially these days, said Aaron Perez, a freshman computer science major. It makes it seem like I've got a chance.

Harvard freshman Madeline Halimi believes Zuckerberg's presence on campus may galvanize another generation of starry-eyed Harvard students.

What's really weird is wondering whether the person next to you will be the next person to invent something that changes the world, Halimi said.