Priscilla Chan knew the man she was marrying would soon be worth billions of dollars but when it came to planning her dream day the now Mrs. Mark Zuckerberg took a decidedly low-key approach.

For one, Chan, a 27-year-old Massachusetts native who has dated the Facebook founder since their undergraduate days at Harvard, chose a wedding gown that costs less than what her new husband likely makes in one hour.

The bride walked down the aisle Saturday, just one day after her husband's company went public and his wealth soared to $19 billion, in a $4,700 dress by designer Claire Pettibone.

The designer discovered that Facebook's first lady wore her dress, featuring a floral sequin-covered overlay, sheer back, and chapel-length train, only after her husband saw a photo of the couple online.

"My husband started reviewing what happened during the day on his tablet and he looked at the picture…and he's like, 'Honey, the bride…she's wearing your dress," Pettibone told " Good Morning America."  "We were just stunned."

Pettibone had no idea that Chan had chosen her ivory, "Sky Between the Branches" gown because Chan used a fake last name to purchase the dress off-the-rack.  The designer and her team searched every shipment they had made of the dress in the last 12 months in order to find where Chan purchased her gown.

"We checked and the Little White Dress Bridal Shop in Denver, Colo., had it and we looked at the order and it was Priscilla with a different last name but the wedding date, May 19, and we thought that's got to be it," she said.  "They [store employees] had no idea who she was.  They thought she was just sweet and nice and lovely."

Pettibone, who described the dress as sexy but elegant and understated, says she is still overwhelmed that Chan chose her design, and guesses the famous bride could be America's version of Kate Middleton, referring to the famous royal newlywed who set off a flurry for her wedding dress after she wed Prince William last April.

"I think this is something that every designer dreams of and talks about over the years…it would be really great to get a high-profile celebrity, but wow," Pettibone   "I still don't understand the whole impact of it."

"I think it'll definitely go up in popularity after this," she said of the dress, which first debuted on the New York runways in October 2010.

Also surprised by the couple were their 100 guests who thought they were at Zuckerberg and Chan's Palo Alto, Calif., home to celebrate Chan's recent graduation from the University of California San Francisco medical school, where she has been studying to be a pediatrician.

"It had that low-key feel," Jen Garcia, senior write for People magazine told "GMA."  "It was traditional.  It was rustic.  They had these great twinkling lights but it was candle-lit and there were very few flowers."

Chan walked down the aisle to Zuckerberg, 28, with her fluffy sheep dog, Beast, and the couple recited their own vows, reportedly grinning the entire time.

Also low-key is the engagement ring Chan has been spotted wearing, a simple ruby reportedly designed by Zuckerberg himself.

"You would expect her to have this huge, 20-carat sparkler," Garcia said.  "The ring itself just speaks to the couple. It's very modest."

The wedding feast came from the couple's favorite Palo Alto restaurants, Palo Alto Sol and Fuki Sushi, and for desert they had Burdick Chocolate "mice," which Chan and Zuckerberg had on their first date.

The couple had dated for nine years before tying the knot.

ABC News' Taylor Behrendt, Dean Schabner and Alyssa Newcomb contributed to this report.