The ever intrepid folks over at MuckRock have succeeded in obtaining the NYPD's social media policy, which is interesting to read.
Officers are urged not to identify their status as police officers. In fact, doing so makes them ineligible for certain assignments (presumably undercover work). They are also prohibited from tagging or identifying their fellow officers, from posting pictures of themselves in uniform and from friending or following minors. They also can't post crime scene photos online.