Buttoned-down social network LinkedIn has reeled in $22.7 million from a trio of new strategic investors in an extension of the $53 million series D round announced in June.

Wall Street bank Goldman Sachs, business publisher McGraw-Hill Companies and SAP Ventures, the venture-capital arm of the German business software maker, joined previous investor Bessemer Venture Partners in the follow-on round.

As with other social networks, the challenge for fast-growing LinkedIn is to turn the more than 30 million users it claims into a stream of revenue and profit. New investors Goldman Sachs, McGraw-Hill and SAP could help build new commercial tie-ins that would accelerate that process. Internet tracker comScore reported that LinkedIn had 8.7 million unique visitors in April, a 361 percent increase.

Chief Executive Dan Nye, writing on the LinkedIn blog, said the latest funding would help create “additional services” for users as they connect and collaborate with their professional networks.

In interviews after the June round, Mr. Nye said a LinkedIn initial public offering was “very likely,” but at a valuation higher than the $1 billion that funding implied. The June funding round was led by Bain Capital Ventures with additional funding from prior investors, including Sequoia Capital, Greylock Partners, and Bessemer.

Social networks have fetched premium prices because the online traffic they generate creates a potential platform for advertisers. In 2005, Rupert Murdoch's News Corp. bought Intermix, the parent of MySpace, for $580 million. In October 2007, Microsoft paid $240 million for a 1.6 percent strategic stake in Facebook that implied a $15 billion valuation.

LinkedIn’s stable of angel investors includes Marc Andreessen, co-founder of Netscape, and Peter Thiel, co-founder of PayPal.