Groupon investors could use a spa treatment discount right about now.

Shares in the world’s No. 1 daily deal company, famous for its half-off offers on everything from spa outings to fencing classes, fell to their initial public offering price of $20 a share yesterday on fears that margins will get squeezed by runaway expenses.

Groupon, which created demand for its stock by selling only 5 percent of its shares, saw a more than 30 percent pop during its debut.

Yesterday those gains evaporated, with shares falling almost 15 percent to close at $20.07 — before nosing down to $20 in after-hours trading.

A number of Web 2.0 tech companies have been struggling since going public, including LinkedIn, whose stock has slipped since a lockup period preventing insiders from selling expired this week, opening the way for an increased supply of shares on the market.