Shares of Facebook Inc. fell $1.68, or 5.7 percent, to $27.92 in midday trading Friday. That's down more than 26 percent from its initial public offering price of $38. And it's down about 12.5 percent for the week.

Facebook began trading on the Nasdaq two weeks ago Friday on a day marred by trading glitches and general investor confusion. The IPO capped the worst week for the U.S. stock market so far this year.

Baird analyst Colin Sebastian started coverage of Facebook's stock Friday with an "Outperform" rating and a target price of $37. The keyword, he says, is potential — in advertising, payments and e-commerce.