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Business schools provided vital connections for previous generations of Silicon Valley start-ups. Now accelerators and incubators—programs where entrepreneurs spend a few months developing their tech concepts—are becoming the alumni networks of choice.

Many founders of angel-funded and venture-capital-backed companies who have gone through top accelerators and incubators like Y Combinator, 500 Startups and Techstars say those networks matter as much or more than their alma maters. Through accelerator alumni, entrepreneurs say they gain access to dozens or even hundreds of...