US market regulator Security and Exchange Commission began examining disclosure rules for privately held companies or whether they need to be rewritten following the recent deals that allowed investors to buy shares in Internet companies like Facebook and Twitter, The Wall Street Journal reported.

The regulators are eager to safeguard the dividing line between public and private companies after investment giant Goldman Sachs pumped in $450 million into social networking site Facebook earlier this week thus triggering a probe, the Journal reported on Wednesday.

Russian investment firm Digital Sky Technologies also invested another $50 million into the social networking site.

The SEC is re-examining the divide between public and private companies following Facebook's agreement with Goldman Sachs Group to create an investment vehicle that will allow some of the securities firm's richest clients to buy as much as $1.5 billion dollars of equity in social networking site, the report said.

SEC rules require firms with 500 or more shareholders of record in a given type of stock to publicly disclose certain financial information. The requirement is designed to protect investors from risking money on companies that say little about their operations and performance.

The review is an early stage and SEC officials, who are examinging the recent deals have not concluded that any of them ran afoul of the existing rules that govern the private companies, the report said citing people familiar with the probe.

Facebook has more than 500 million active users a month worldwide and its chief executive Mark Zuckerberg, 26, was named Time magazine’s “person of the year” even as he is resisting pressure to take his company public.

Facebook has a capitalization, which is more than that of Boeing, at $48.7 billion or Time Warner at $36 billion following the fresh investment. However, Facebook’s annual revenue based on advertising is estimated at $2 billion compared with Boeing’s $64.62 billion and Time Warner’s $26.5 billion.