SAN FRANCISCO—Internet start-ups across Silicon Valley are struggling to compete for talent amid the investment frenzy gripping Facebook Inc., Twitter Inc. and Zynga Inc., with many smaller companies beefing up pay and recruiting and wading into the private-company share market to keep pace with their larger rivals.

Online real-estate brokerage Redfin Corp. is feeling the fallout. The 200-person company, which is based in Seattle and has offices in San Francisco, typically hires new engineers fresh out of college, relying on competitive compensation and the allure of working at a profitable start-up.

Redfin said it has recently been up against salary-and-bonus offers of $100,000 to $150,000 a year for new college grads from social-gaming start-up Zynga, among others—far above the $80,000 or so a year Redfin would normally offer.

“Zynga and others have introduced such great financial incentives that it’s hard to win candidates anymore,” said Glenn Kelman, Redfin’s chief executive.

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