It is a good idea to let staff know about the investment. They might want to invest themselves, or they might have ideas of how to amplify the message.
This very much depends on the channels available. Look to the other pages in this section for guidance of how to execute comms for particular situations. In a lot of ways, the messaging needed for staff is no different from broader resident communications, so it is good advice to treat it as such.
Just because this content is for staff does not make any difference to risk warning requirements. You should follow the guidance that relates to the channel you are looking to use. For the avoidance of doubt, whatsapp and other direct messaging services fall under social media.