SAN FRANCISCO (MarketWatch) — Tesla Motors Inc. Chief Executive Elon Musk is not afraid of pushing the envelope — either in business or in the fast-evolving world of social media.

But Musk’s tweets about the electric-car company raise a fresh round of questions about how and when social media can be used to divulge market-sensitive information to the public.

More companies are using social media to disseminate their news — it’s cheap, fast, and tailored to reach millennials, a prized demographics.

Earlier this week, Musk lit up Twitter with a message about a “major new Tesla product line.” Tesla shares jumped, almost instantly boosting the company’s market capitalization by more than $1 billion.

Tesla TSLA, +4.42% shares got another pop when Musk and Tesla’s main Twitter feed highlighted an April Fools’ joke launched on the company’s website.

Most chief executives and entrepreneurs, however, don’t announce corporate news via Twitter Inc. TWTR, +2.03% . On Twitter, they typically react to general news, comment on news about their companies after the fact, or offer a bit of color about their day-to-day lives and whereabouts.

Some are not active at all on social media. Warren Buffett, the chairman and chief executive of Berkshire Hathaway BRK.A, +0.36% , joined Twitter two years amid much fanfare, but only has six tweets to his name (and nearly 1 million followers). Apple Inc. AAPL, -3.17% Chief Executive Tim Cook has fewer than 150 tweets (and more than 1 million followers).

A Tesla spokesman said the company “carefully” follows SEC requirements, and remains focused on “long-term results and on delighting Model S owners everyday.”

The SEC rule, known as the Regulation Fair Disclosure, or Reg FD for short, forbids selective disclosure of corporate news. Two years ago, the SEC clarified that companies may add social media to their arsenal of disclosures and announcements, but only if certain conditions are met.

When fax machines roamed the earth

Reg FD was first adopted 15 years ago, when Internet commerce was in its infancy. Back then, hunting down corporate filings using the SEC’s EDGAR online search tool was cutting edge, and some companies were still sending out press releases via fax machine. The SEC updated its rules regarding the use of web sites in 2008 and again in 2013 to include social media.

Regardless of the medium, one rule remains constant: the SEC insists companies cannot disclose information just to certain groups, by design or by accident, without disclosing it to everyone.

A tweet alone reaches a limited audience and can provide market-moving news that does not reach the rest of the market; thus “it needs to be accompanied or preceded by disclosure that reaches the broader world,” said in an e-mail John Coffee, a Columbia Law School professor.

The SEC’s report on social media, in April 2013, was prompted by a casual post in 2012 on the personal Facebook page of Netflix Inc. NFLX, -0.05% chief executive Reed Hastings about Netflix’s viewing numbers.

The SEC ruled that companies can use social-media outlets such as Twitter and Facebook to make announce key announcements, with the caveat “as long as investors have been alerted about which social media will be used to disseminate such information.”

A SEC spokeswoman declined to comment specifically about Musk’s latest tweets. The commission hasn’t brought any Reg FD cases specifically involving disclosure via social media since the 2013 report, she said. And she said the SEC currently has no plans to update its 2013 guidance.

The SEC has brought at least one Reg FD case since then, but it involved phone calls, not social media, she said.

All SEC investigations are conducted privately.

In Musk’s case, among the things the SEC could consider are whether tweets to the two million people following him on Twitter would count as proper disclosure, and whether the company has taken other steps to adequately ensure the public is notified of any information that could affect Tesla’s share price, said Darren Freedman, a corporate and securities lawyer with TroyGould Attorneys in Los Angeles.