I've repeated over & over the dangers of improper electronic communications and how they can come back to bite you. But this article from the latest edition of Inside Counsel does an outstanding job of illustrating how your – seemingly innocent – words may be used against you in a dispute.
If you don't yet have a corporate policy, portions of the article may be an excellent jumping-off point. Do I think the goal is somewhat utopian? Yes, since realistically it would be next to impossible to control the social behavior of everybody under your umbrella. But that doesn't mean a policy shouldn't be crafted and properly promoted in the employee handbook.
There's nothing wrong with a good-faith attempt; especially where ESI is concerned. Perhaps if more people are made to understand the consequences of their actions (most do not imagine sitting in a courtroom answering questions to be one of them), they'll think twice before sending.