Think Mark Zuckerberg doesn’t value privacy? Try getting into Facebook headquarters. Even with an invitation, it’s not easy. When this reporter arrived for a press briefing Wednesday, I got checked twice at the door — and later, security guards almost wouldn’t let me through the building.

The Facebook founder and CEO guards his personal life just as closely. A soon-to-be-released movie, starring Justin Timberlake, makes him out to be a party animal — and he’s obviously not thrilled about it. (“My life is a lot more boring than that,” he told me with an eye roll.) And Zuckerberg thinks carefully about his own Facebook privacy settings; his photos from a recent family vacation, for example, aren’t public. “I wouldn’t have posted those pictures,” he said in an interview, “if I thought the whole world would see them.”

So Zuckerberg cares about his privacy. The question is, does he care about everyone else’s?

The answer, Zuckerberg insists, is yes. But as he revealed in his remarks at Facebook headquarters Wednesday, his definition of privacy might be different from yours.

Mark Zuckerberg has a vision of the future where we all keep vast amounts of data on Facebook, and each of us maintains precise control over what gets shared. Right down to every photo album and status update, we can decide who gets to see what. We can even decide whether web sites can link into Facebook’s “instant customization” feature, and use our Facebook data to customize their content just for us.

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In theory, this is wonderful.

As Zuckerberg says, the Internet is indeed a better place when it’s personal. Sites like YouTube (GOOG), Flickr (YHOO), and Yelp have shown that personal comments and reviews make things more interesting; even Apple (AAPL) has jumped into the personalization game with genius recommendations on iTunes. An Internet experience that’s more personal should be warmer and more engaging; we’ll see news stories our friends think are important, songs they like, restaurants they recommend.

So what’s the problem?

Zuckerberg outlined it pretty clearly Wednesday: Extremely private people are bad for Facebook’s growth. As Facebook prepares to grow to 500 million accounts and beyond, it has to invent new ways to get us to broadly share our information. Zuckerberg doesn’t just want us connecting with our friends — he wants us sharing our thoughts and interests across the Internet, with people who haven’t signed into Facebook yet. As that happens, it entices new people to join and brings us closer to the goal the Facebook founder states in the bio of his profile: “I’m trying to make the world a more open place.”

That’s why we’re guaranteed to see Facebook at the center of another privacy crisis before long. Ever since Zuckerberg first opened the service beyond its core group of college students, a core group of users has complained that he was breaching their trust and ruining the network. Practically every change, including redesigns, real-time updates and the News Feed, provoke a minor backlash.

The best Zuckerberg can hope? That he can react quickly enough when users tell him he’s pushing his “open” agenda too far. Because one thing’s for certain — he’s determined to keep growing Facebook — and that often means pushing users beyond their privacy comfort zones.

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