Over the past couple of years, a herd of unicorns — the tech industry’s term for private companies worth more than a billion dollars — have thundered through Silicon Valley. This week, one of them stumbled out of the pack.

Brisbane’s Mode Media has abruptly shut down, leaving bloggers unpaid, investors frustrated and rumors swirling in its wake.

The company, a publisher of lifestyle sites and operator of an ad network which placed big brands’ campaigns on smaller websites, told investors and employees Thursday that it was closing its doors after failing to find a buyer or line up financing.

Bankruptcy appears to be a possibility. It would be the first so-called unicorn based in the Bay Area to face such a fate. While others have been gobbled up in acquisitions or forced to accept lower valuations in financing rounds, none of the companies publicly tracked on lists of unicorns has simply gone under.

Only one small part of the business, a provider of private social networks called Ning, will continue to operate, thanks to a deal with a company run by a Mode board member.

Samir Arora, a showy entrepreneur who made his name by selling a software company to IBM during the dot-com boom, started Mode as Glam.com in 2003. The startup became a publisher for fashion and lifestyle bloggers, many of whom now allege the company failed to pay them for ads it ran on their sites.

As Glam, the company sought female audiences with a pink logo and splashed the color on its office walls, and Arora dressed in colorful ties or pocket squares to match. After the company changed its name to 2014 in Mode, courting wider audiences, it adopted a black-and-white design, and Arora adopted an all-black wardrobe, Forbes reported last year.

The company raised at least $224.6 million from 10 investors including Accel Partners, Draper Fisher Jurvetson, German company Hubert Burda Media and others. It also received debt financing from Silicon Valley Bank.

As talk of an initial public offering grew, Mode acquired social network Ning in 2011. The deal brought Marc Andreessen, a Ning co-founder, onto Mode Media’s board of directors.

An August 2013 round of funding valued the company at $1 billion. According to a report in Business Insider, the company filed secretly for an initial public offering that year, but never followed through.

The company had offices around the world, including Los Angeles, New York, London, Mumbai and Tokyo. A search on LinkedIn shows 361 people who list Mode Media as their current employer. Employees were not paid severance, according to one person who was laid off Saturday.

As an ad network, Ning sold ads and placed them on its own websites as well as independent ones with which it had deals. It then collected money from companies and ad agencies, sometimes months after the campaigns ran, and then paid out a share of the revenue to the independent publishers it represented. As such, at any given time, it owed and was owed a substantial portion of the money it made.

Mode experienced turnover at the top in the following years, culminating in the departure of founding executives and board members this year.

In April, co-founder and CEO Arora resigned and Andreessen left the board. Jack Rotolo, a top sales executive, stepped in as interim CEO. A source close to the company said the events leading up to Arora and Andreessen’s departures were an early sign of trouble.

A spokeswoman for Andreessen Horowitz confirmed the timing of Andreessen’s board departure and said the firm didn’t have more information on Mode.

Co-founder Dianna Mullins departed in August. She wrote a farewell letter addressed to the company on Medium that foreshadowed its surprisingly rapid end.

“You had it all,” Mullins wrote. Then, she continued, “Success started to wear on you. You started to go a bit wayward.” Mullins declined to comment further.

On a company blog, Mode’s Ning subsidiary announced “exciting news” Thursday that Cyndx LLC will “take over the operations of ... Ning Inc. from Mode Media effective immediately.”

Cyndx is a New York investment-advisory firm run by Jim McVeigh, a former Bank of America Merrill Lynch media and technology banker. A biography of McVeigh provided by Monster Worldwide, where he serves as a director, says that McVeigh is a member of Mode Media’s board of directors.

Reached Friday, McVeigh declined to comment on whether he is still on Mode’s board or discuss the company’s sudden collapse.

Unlike Mode’s advertising business, Ning made steady if small revenues from the thousands of customers who used it for private networking. Only three people present themselves as working on Ning on their LinkedIn profiles, though others at the company may have played support roles.

James Armstrong worked as senior director of technical operations at Mode Media between 2007 and 2009. He said he noticed an unusual amount of turnover in his time with the company.

“At least half the people who were there when I started were not there when I left,” Armstrong said.

Armstrong said conversations he’s since had with other employees showed signs of trouble.

“It seemed they were spinning their wheels and not building a business anymore,” Armstrong said. “Their moment had passed.”

Jessica Floum is a San Francisco Chronicle staff writer. Email: jfloum@sfchronicle.com Twitter: @jfloum