Groupon shares jumped more than 50% to almost $31 in its public debut Friday, showing strong demand for a company whose business model is considered unsustainable by some analysts.

Chicago-based Groupon sends out frequent emails to subscribers offering discount deals on anything from laser hair removal to weekend getaways. The company takes a cut of what people pay and gives the rest to the merchant.

Though it spawned many copycats after its 2008 launch, Groupon has the advantage of being first. This has meant brand recognition and investor demand, as evidenced by its sizzling public stock debut.

Groupon is selling 5.5% of its available shares. Though not unprecedented, that amount of publicly traded stock is below the so-called "float" for many prominent tech companies, such as Google and more recently LinkedIn.

On Thursday, the company priced its IPO at $20 a share. That was above its expected range of $16 to $18. The IPO valued Groupon at $13.3 billion and raised $700 million.

With Friday's stock price jump, Groupon's value rose to more than $18 billion.

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