Shares of high-tech real estate brokerage Redfin surged on their first day of trading Friday. Initially offered for $15 per share, the company’s stock had soared above $20 by early afternoon, valuing the company at more than $1.5 billion.

Founded in 2004, Redfin pioneered the concept of putting real estate listings on an interactive map alongside rivals Trulia and Zillow. But Redfin chose a different, more ambitious business model than its competitors. Zillow and Trulia make money by referring customers to independent real estate brokers. Redfin, in contrast, aimed to disrupt the real estate business by becoming a real estate brokerage itself.