I am in charge of a team at work, and every time the company sends a survey I emphasize the same point. I strongly believe that in a real survey there is no right and wrong (I'm talking surveys about how you feel regarding certain subjects), yet as we all know since we're in the internet right now, anonymity gives people a huge sense of security and disregard for potential consequences, so the idea of anonimity can make people see a survey as a blank slate to vent, joke or throw insults around.

Always assume any survey from your company is NOT anonymous, keep it honest, but keep it respectful.