Dropbox, the file-sharing company and Silicon Valley darling, had a strong market debut Friday, a reassuring sign for the technology industry and for the investors who have billions locked up in other highly valued but privately held startups.

Shares of San Francisco-based Dropbox, which had sold 36 million shares at $21 each on Thursday night, rose 36 percent in the first day of trading. That pushed Dropbox’s market value to $11.2 billion, above where it had been valued in the private markets.

The initial public offering arrived at a dicey time. Stock markets have been on a roller coaster, with investors anxious about the threat of inflation, possible trade wars and geopolitical risks. The technology industry in particular has been buffeted by the prospect of tougher government regulation.

Giant tech companies helped drive another sharp sell-off Friday. The S&P 500 tumbled 2 percent, with both Facebook and Amazon falling more than 3 percent. The S&P is down more than 3 percent this year.

But the strong reception for Dropbox, which sells subscriptions to software that lets users collaborate and share files online, suggests that the public markets remain willing to offer a crucial exit point for early-stage investors in the largest private Silicon Valley startups, even amid broader concerns about large tech companies.

Over the last decade, hundreds of billions of dollars have poured into the venture capital industry. The influx of cash fueled the tech economy.

Venture capital money meant that highly valued startups — “unicorns,” in Silicon Valley parlance — were able to remain private for longer without turning to the public markets for fundraising. That posed a risk for venture capital firms and other investors, whose stakes had gained large amounts of value on paper but weren’t easily liquidated.

The public markets enable the companies to raise money to accelerate growth and, sometimes just as important, to provide a way for their early investors to cash out.

As a result, the offering for Dropbox is good news for investors with large gains locked up in private companies.

Drew Houston and Arash Ferdowsi founded Dropbox in 2007. It has never turned an annual profit, but strong sales growth in recent years has pushed revenue above $1 billion annually.