So recently, I've played in a game (won't mention any game or personal details) where at some point, screenshots of diplomatic conversations were sent to third parties. Of course, these conversations were embarrassing, revealing complicated schemes to stab other players.
In my case, I am player A, player B sent me a screenshot in which player C apparently revealed sensitive information about my moves to B and was planning further cooperation with B to destroy me, A.
Of course, I immediately sent the screenshot to player C, telling him "look what player B did, revealing sensitive information and everything", and I said I immediately forgave him the fact that he was plotting against me if he would change sides, which he did.
The point is that this is a case of meta-gaming. You are using information from outside the game (the screenshots are placed at some "imgur" website) to influence what is happening inside the game (breaking of trust between two players to take advantage of the ensuing war).
What is the difference between sending a screenshot and copy pasting diplomatic post? The difference is huge: deniability. In case of a screenshot, you can't deny that you actually wrote it because the information is simply there, on the screen. When information is copy pasted (a practice that I am in favor of) there is always the option of denial. It's like saying "Mrs. Jones, your husband is cheating on you with me" or "Mrs. Jones, your husband is cheating on you with me and here are a couple of photos of us having sex."
In a FtF game, the equivalent of this practice would be to bring audio-recording material and let other players listen to the delicate parts.
I've arrived at the point where I simply cannot send diplomatic information to certain countries - not for fear of them telling the others - but for fear of them taking screenshots of it and showing the others. This is not how I want to play Diplomacy. I hope the webdip-community speaks out strongly against this practice and the mods clarify that this is, in fact, a case of metagaming.
No, this is not a cheating accusation. I'm not mentioning a particular player, or country, or game, and I am not revealing whether the game has finished or is ongoing. What I can say is that this particular game is ruined, there's no more fun in it.