Rosenstein dropped out of a graduate program in computer science at Stanford in 2004 to join Google as a product manager. At Google, Rosenstein led projects in Google's communication and collaboration division. His projects initially included Google Page Creator, the precursor to Google Sites, and a project internally codenamed “Platypus,” which eventually became Google Drive. He also invented and wrote the original prototype for GMail Chat  and many of the features in Google’s rich text editor.
In May 2007, Rosenstein left Google to become an engineering lead at FaceBook, working closely with Mark Zuckerberg and Dustin Moskovitz. He was technical lead in charge of Facebook's Pages, the Facebook Like button, and Facebook Beacon. He was compensated with 239,165 Class B shares ($16 million at $70/share), which he deposited into a trust.