They could work the irony by having "Whisper" in large font and then "... don't shout" in 75% or 50% font next to it. Would work better as a metaphor because (

They could work the irony by having "Whisper" in large font and then "... don't shout" in 75% or 50% font next to it. Would work better as a metaphor because (I guess) this statement/site is implying that "Whisper" is the more effective communication approach.