WSN includes a simple instant messaging system which you can enable in your switches. User profiles and the online users list will have icons to click to start a conversation. Conversations open in a small popup window, which refreshes an iframe regularly to check for new messages. A member with IMs waiting will get a message box prompt to open a conversation. If the member is no longer online, they'll get the message when they return.

Once the switch is enabled, some settings are available for configuration at Admin -> Settings -> PM/IM.