And, of course, transforming media and politics by presenting news and opinion -- true and fake -- to billions of people around the world.
Now the Facebook CEO is acknowledging that connecting people online isn't enough.
"We used to have a sense that if we could just do those things, then that would make a lot of the things in the world better by themselves," Zuckerberg told CNN Tech. "But now we realize that we need to do more too. It's important to give people a voice, to get a diversity of opinions out there, but on top of that, you also need to do this work of building common ground so that way we can all move forward together."
The company even has a new mission statement: "To give people the power to build community and bring the world closer together."
This marks the first time the company has overhauled its mission, which had previously been "to give people the power to share and make the world more open and connected."
Zuckerberg believes he has just the tool for the job: Facebook Groups, which are now used by a billion people.
"A lot of what we can do is to help create a more civil and productive debate on some of the bigger issues as well," Zuckerberg told CNN Tech's Laurie Segall in Chicago on Wednesday night. It was his first in-depth interview for television since 2012.
Facebook (FB, Tech30) debuted the new Groups features at the start of its first "Communities Summit" in Chicago. Three hundred Facebook Group administrators from across the country are attending the two-day event to hear speeches from Zuckerberg and other executives, and attend panels on topics like conflict resolution.
The administrators represent the variety of groups that have sprung up on Facebook, including Mormon mothers supporting their gay children, people suffering from rare diseases, and mental health support for veterinarians, who have a surprisingly high rate of suicide.
On Facebook: 'I think we're doing OK'
The new emphasis on Groups is the culmination of months of public appearances and posts by Zuckerberg stressing the importance of community. He's been traveling around the U.S., professional photographer in tow, to meet people from every state. During a commencement speech at Harvard, Zuckerberg touched on hot-button topics like immigration and inequality.
Despite the political tone of the events, Zuckerberg has said he's not running for office. He's sticking to making an impact with the world's most ubiquitous social media platform and its almost 2 billion users.
Zuckerberg tends to phrase things Facebook does in terms of their benefit to humanity, not investors or the company's bottom line. Facebook, which has a market value of about $440 billion, has a responsibility to use its sizable resources to do positive things, he says, and that should naturally create value down the line.
"That's why it helps to have control of the company," he said. (Zuckerberg maintains the majority voting rights at Facebook.)
Facebook has more than 1.9 billion monthly users. It reported $8 billion in revenue last quarter, nearly all of which was from ads. Or, as Zuckerberg puts it, "I think we're doing OK."
But the company has struggled with a number of issues over the past year, including the proliferation of "fake news," live broadcasting of murders, and creating filter bubbles that have contributed to a divided nation. It's part of what pushed Zuckerberg to reexamine Facebook's mission, starting with a 5,726 word post on "Building Global Community" in February.
When Facebook launched Groups in 2010, it was a barebones tool for sharing updates with people you knew personally, like family members or a kid's softball team. Now there are groups for every interest imaginable, with a few members or hundreds of thousands of people. Anyone can start a group, and they can be public, closed but publicly listed, or hidden. An administrator can control who joins and police content.
The way Zuckerberg describes it, being active in Groups can be a political act, a grassroots way to address the ills of the world.
"A lot of the important stuff that needs to happen in the world [is] global ... whether that's addressing challenges like stopping climate change or stopping pandemic diseases or terrorism," said Zuckerberg. "None of these things can happen just by one country or group of people deciding to do it. There's no top-down structure to enable that, so the will needs to be built bottom up."
A billion users someday for 'meaningful' Facebook Groups
A Facebook Group may not be able to stop global warming, but a healthy fear of social media can have an impact on decision makers.
A local Facebook Group in Miami called "Clean Up Miami Beach" has gone from coordinating litter cleanups of 10 people to a political force with more than 4,500 members. Administrator Michael DeFilippi has the ears of local politicians -- Miami's mayor and city manager are members -- and successfully pushed for new legislation to outlaw Styrofoam.
"Because of Facebook, and because it's free, it gives you a level playing field with forces that can pay to get marketing and lobbying out there," said DeFilippi.
Only 100 million Facebook users are in what Zuckerberg calls "meaningful" groups -- the kinds that become part of your support structure. He says creating these virtual communities is increasingly important as membership in real world organizations like unions, churches, and PTAs decline.
The company is using artificial intelligence to recommend more of these types of groups. It's also adding tools to help group leaders screen new members, block people, schedule posts and link groups together.
"I think [it] gives us a good shot within five years or so to get to this goal of connecting a billion people to meaningful communities," said Zuckerberg.
Some of those communities could not have existed before the internet.
Brian Copeland, who lives in Tennessee, runs a group for gay fathers. There are members from around the world, including remote areas where there are no other gay parents nearby and countries where the fathers are fearful of being discovered.
Its members meet up in the real world to vacation together at Disney World and Dollywood, says Copeland. They also help each other on bigger issues. Members exchange legal advice and some have even used the group to help foster children find permanent homes.
Free speech, but not 'hate speech'
Bringing people together based on shared interests isn't good by default. Racism, bullying and radicalization can thrive in closed groups populated by like-minded people. There are large Facebook communities for White Nationalist organizations and other hate groups.
Facebook has started using artificial intelligence to identify users or groups trying to engage in terrorist recruiting, and Zuckerberg says they'll do more over time as the artificial intelligence gets better. But he does not want Facebook to be in the business of policing content.
"An important aspect of freedom of speech is that you need to be able to get pretty close to offensive," said Zuckerberg. But bullying or anything that causes real world harm is off limits. Disagreeable content is allowed, "as long as it's not hate speech or way over the line."
Even as it charges ahead with a shiny new mission to connect the world, Facebook will still have to deal with the darker side of humanity.
"Facebook's a parallel universe to the real world, and for us to think hate and vitriol is not going to exist, or to think that love and acceptance isn't going to exist, is unrealistic," said Copeland. "[But] Facebook has a stewardship responsibility ... to keep those areas safe for everyone."