Pfizer (NYSE: PFE) spinoff Zoetis (NYSE: ZTS), which includes what used to be Embrex in North Carolina, has debuted on Nasadaq in the largest offering since Facebook (Nasdaq: FB). Embrex was a Durham-based agriculture biotech company with 300 employees when it was acquired by Pfizer in 2007. Pfizer is divesting its non-pharmacuticals units so it can focus on its core prescription drugs business, a move that brought the sale of its infant nutrition division to Nestle for $11.9 billion last year. Madison,…