Facebook Inc.’s growing ambitions are redrawing battle lines in Silicon Valley.

As the seven-year-old company ramps up its hiring and adds new features to its social network, it is disrupting the businesses of established companies like Yahoo Inc. and Google Inc. and putting even more Internet firms on notice.

Facebook, which has more than 600 million users and was valued at $50 billion in a recent funding round, is grabbing online-advertising from Yahoo, Myspace and others. The social network is a potential rival in electronic payments to eBay Inc.’s PayPal, while partnerships Facebook is cementing with smartphone makers set the stage for competition with Apple Inc. and Google in mobile services.

Meanwhile, Facebook is tussling with Google and Microsoft Corp. for top engineers.

As a result, many Silicon Valley companies increasingly have to decide whether to treat Facebook like a friend whose reach and user data can help propel their own growth, or a foe that can become a destructive force.

“Facebook is both a great competitor and a benefactor here in Silicon Valley,” said David Cowan, a venture capitalist at Bessemer Venture Partners in Menlo Park, Calif. “Anyone who’s trying to get the attention of the young Internet user now has to compete with the dominant position that Facebook has there. On the other hand, they have opened up a lot of opportunities.”

Facebook executives aren’t shy about their aspirations. “We think every industry is going to be rebuilt around social engagement,” Chief Operating Officer Sheryl Sandberg said.

Facebook already helped spur a new crop of videogame companies designed around interacting with friends, Ms. Sandberg said, adding, “News, health, finance, shopping and commerce—we think similarly, all of these things will be rebuilt by companies that work with us to put social at the core.”

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