Put it on sharepoint and it can get to squadrons, but relies on squadrons promulgating that info.
Put it on social media and you spread the message faster and wider, reaching more people and helping to ensure a consistent message.
Of course they need to ensure the message is accurate, but the mechanism is sound for things like this. The poster (and any similar ones) is a quick aide memoire to support policy promulgated in the usual manner. This poster in particular was initially designed to try and prevent squadrons thinking they know what they are doing but being too lazy to check directly.