Little-known social coding start-up GitHub Inc. has raised $100 million in its first round of funding, in a sign of how big investment bets are continuing in Silicon Valley.

The round also marks the biggest check written to date by venture-capital firm Andreessen Horowitz, which is leading GitHub's financing and putting in more than 99% of the money. The funding values four-year-old GitHub at $750 million, said a person briefed about the deal.

"It's the largest investment we've ever made" as GitHub tries to become the standard in software coding, said Andreessen Horowitz venture capitalist Peter Levine, who joins GitHub's board.

GitHub's funding stands out amid concerns over the start-up investment environment after Facebook Inc.'s disappointing initial public offering in May. While some venture capitalists said they have since reconsidered jumping into high-priced late-stage funding rounds, many venture firms continue pumping money into younger start-ups and are also raising new funds.

On Monday, Dow Jones VentureSource reported that 82 venture funds collected $13 billion in the year's first half, up 31% in capital from a year earlier.