By Kristen Mascia Johnny Dodd
June 04, 2012 12:00 PM

Mark Zuckerberg hadn’t told his nearly 100 guests what was happening. In their minds it was a belated fete to celebrate his girlfriend Priscilla Chan’s May 14 medical school graduation. So when Chan turned the corner in their Palo Alto backyard on May 19 wearing a white gown and escorted by their trusty Hungarian sheepdog Beast, the small crowd gasped, then cheered. “It was really cute,” says a guest authorized by the couple to speak to PEOPLE. “Naturally people were surprised but happy.”

So much for the billionaire bachelor! Just a day after Facebook went public in one of the most anticipated stock offerings ever, Zuckerberg, 28-now worth an estimated $17 billion-turned another milestone, wedding Chan, 27, in a ceremony they planned for seven months. Joined by their “family and closest friends,” the guest says, the couple of nine years couldn’t stop grinning. Not that anyone was totally shocked. Though some questioned the curious timing of the marriage after Facebook’s IPO (see box), “their relationship is based on two people before all this craziness went down,” his oldest sister, Randi, told PEOPLE in 2010.

So who is the newly minted bride? Trilingual in English, Spanish and Cantonese, the Braintree, Mass., native and recent University of California, San Francisco medical school graduate is no wallflower, for one. Understated and “hugely committed,” says a source, “to education and her career,” she’s also a lover of the simpler things in life, describing herself on her own Facebook page as a fan of “cooking and soft things” and “the food network, warm places, sun dried tomatoes and diet a&w.” Since meeting in line for the bathroom at a party at Zuckerberg’s Harvard fraternity in 2003, the hardworking pair seemed destined to be together. “He was this nerdy guy who was just a little bit out there,” Chan told The New Yorker. Though she once joked, “I’d love a job at Facebook,” the 2007 Harvard grad chose instead to teach elementary school for a year in San Jose, Calif., where, after a break with Zuckerberg, she joined him. There they built a low-key life, tooling around town in his less-than-flashy Acura and spending lazy days strolling Target. “It’s so funny because you wouldn’t think Mark Zuckerberg’s girlfriend would be into that,” said Randi.

As Zuckerberg’s public profile grew-especially with Aaron Sorkin’s punishing big-screen take on Facebook, The Social Network-Chan stuck by him, but friends say not for the payout. “They share the same values and love and respect each other,” said Randi of the couple. In fact Chan, who starts her pediatric residency soon, motivated Zuckerberg to implement a new organ-donor feature on Facebook.

She also inspired the son of a dentist from Dobbs Ferry, N.Y., to take up Chinese, which came in handy on the morning of the wedding when he told Chan’s grandmother about the top-secret ceremony. That afternoon, wearing the ruby ring Zuckerberg designed and a sleeveless gown by Claire Pettibone, Chan stood with Zuckerberg and exchanged vows they’d written themselves. After dishes from their favorite local haunts Palo Alto Sol, a Mexican restaurant, and Fuki Sushi, they finished the night with a taste of nostalgia: mice-shaped chocolates from Burdick Chocolate, sweets they’d had on their first date. By the next day, both had logged on to Facebook to change their status to “Married.” “Billions or no, they’ve always stayed down to earth,” a source close to them told reporters May 21. “They’re a good fit.”