Last year, a giant hole appeared in Menlo Park, Calif.

Neighbors were told it was a foundation for a new house. But the pit was so large and deep that some wondered whether it was a subterranean condo complex rather than a home. When construction crews recently started hoisting huge steel beams on the property, residents grew more concerned.

The home, still under construction, is expected to be one of the largest in Menlo Park, stretching to over 9,200 square feet. With its futuristic glass and steel exterior, the mansion will mark a dramaticcontrast to the traditional shingled ranch homes and Spanish villas nearby.

It will also stand out because of its owner: Sheryl Sandberg, the high-profile Facebook COO and one of Silicon Valley's newest paper billionaires.

'She can build whatever she wants to,' said Ron Shepherd, an insurance broker who lives nearby. 'But that kind of modern house just doesn't fit with the neighborhood.' By billionaire standards, Ms. Sandberg's mansion is nothing special and some neighbors don't mind it. Yet the house has quickly become a monument to the new, new money of social media, and it shows how today's IPO wealth is literally changing the Silicon Valley landscape.

Facebook's offering, expected this spring, is likely to mint multiple billionaires and create hundreds if not thousands of new millionaires. The infusion of new wealth could pose new challenges to the company, known for its jeans-and-hoodie culture. 'We don't wake up in the morning with the primary goal of making money,' the company said in its IPO filing.

The IPOs of Zynga, LinkedIn, Groupon and other social-media companies have converged to form a wealth wave that, while more concentrated, is drawing comparisons to the dot-com boom of the late-1990s.

The sudden riches are already trickling down to everything from cars and jet charters to restaurants, ski trips and adventure treks in New Zealand.

Yet the most visible gains are in real estate. Home prices in parts of San Francisco and parts of Silicon Valley are soaring.

Yuri Milner, the Russian tech investor who put millions into Facebook, Zynga and other social-media companies, last year bought a $100 million mansion in Los Altos.

Dick Breaux, who owns Peninsula Custom Homes, says he is working on four large new homes for clients in the tech industry. One is a 14,000-square-foot house in Hillsborough, which includes a guesthouse, an amphitheater and reflecting ponds.

Architect Bob Swatt, of Bay Area firm Swatt Miers, says he is busier than usual with 10 homes that range from 6,000 to 9,000 square feet, six of which are under construction in Silicon Valley for clients in the tech field.

Menlo Park -- a city of 32,000 that is home to Facebook's new headquarters -- has received applications for construction for 15 new homes since August and has approved four applications for home demolitions.

Five of the homes under construction are more than 6,000 square feet. Most homes in Menlo Park, long known as a community of Stanford professors and young families, are around 2,000 to 3,000 square feet.

At Facebook, Ms. Sandberg is often seen as the 'adult supervision' of the company, imposingmanagementdiscipline and practical profit goals. The 42-year-old Harvard- and Harvard Business School grad rose from being an economist at the World Bank to working at the White House, as chief of staff for then-Treasury Secretary Larry Summers. In 2001, she became Google's vice president for online sales and went to Facebook in 2008.

Ms. Sandberg may soon become one of America's richest self-made women. According to Facebook's IPO filing, she has more than 1.8 million Facebook shares and 39 million unvested options and restricted stock that could give her a net worth of more than $1.5 billion based on expected valuations of the company. Ms. Sandberg's husband, David Goldberg, is CEO of SurveyMonkey, an online survey company.

Ms. Sandberg is actually downsizing. The family currently lives in a 7,120-square-foot home, with a 4,580-square-foot finished basement, bringing the total to almost 12,000 square feet, in the nearby super-rich town of Atherton. Ms. Sandberg frequently hosts dinners for Silicon Valley women at her home, with speakers such as Condoleezza Rice, Gloria Steinem and Michael Bloomberg. Last fall, she hosted a $38,500-per-plate fund-raiser for President Barack Obama at her home.

In 2009, she and Mr. Goldberg bought the new property -- a 2,900-square-foot house with a little more than a half-acre -- in Menlo Park for $2.9 million, according to city records.

After going through necessary approvals, the couple tore down the existing house and started construction on the new one. City officials said the owners had permits to remove four trees, including a redwood.

According to plans filed with the city of Menlo Park, the house will have six bedrooms and six baths. The basement houses a wine room, gym, theater, office and guest bedroom. The first floor has a great room stretching over 800 square feet.

Menlo Park zoning rules limit the size of homes in different areas. On Ms. Sandberg's lot, the limit is 6,880 square feet of living space. By digging underground and building a 2,748-square-foot basement, which is excluded from the square-foot limit, Ms. Sandberg and Mr. Goldberg were able to get to 9,210 square feet -- and keep much of the house from public view. The house is 40% larger than Facebook CEO Mark Zuckerberg's.

None of the neighbors interviewed said they had met or talked to Ms. Sandberg or Mr. Goldberg. And some have no problems with the Sandberg mansion. 'There is always construction on this street and their construction hasn't bothered us at all,' said Heather Frauenhofer, who lives nearby. 'It is modern, and there aren't other modern houses around.'

Mr. Shepherd, the other neighbor, says he has nothing against new tech wealth. But he may be less tolerant of visits from the President.

'I'm not an Obama guy,' he said. 'And I would be pretty annoyed if our street is closed down for a bunch of Washington bigwigs.'

Robert Frank / Nancy Keates