Get the functionality of Facebook type messaging on your website without using any third party plugin. WPMU’s Messaging plugin is a complete package in itself which integrates the messaging feature in your WordPress Multisite within few seconds. You can send the message to any user across a network just by using his username. The plugin is simple and light but is very useful for users.
This is the EXACT same plugin as distributed by the developer on WPMU DEV. The developers’ links are provided above.