NEW YORK — Making medicines for livestock and pets may not be as sexy a business as social networking, but Pfizer’s animal health unit is poised to enjoy the biggest stock market debut since Facebook’s.

The Pfizer division, Zoetis, raised $2.2 billion in its initial public offering Thursday, exceeding expectations by pricing its stock at $26 a share, above the expected $22 to $25, pricing the company at $13 billion.