At Admin Panel -> Settings -> Switches there's an "instant messaging" option. WSN includes a simple instant messaging system which you can enable with that. When activated, user profiles and the online users list will have icons to click to start a conversation. Conversations open in a small popup window, which autoloads new messages every few seconds. A member with IMs waiting who doesn't have an IM window open yet 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. There's also a template for the instant messaging window at Admin -> Themes -> Manage Templates.