The private messaging feature works like an internal site email similar to the Inbox in Facebook. Each member has their own individual Inbox, Sent, and Compose page. You are notified of new messages by the Notifications tab in your admin bar as well as by email notification (if enabled).
The biggest difference between Facebook and eResources Library is that you don’t have to be a friend to send a private message; provided you know a member’s username you can send a message to any person on the network.
However, being friends makes the process easier since like in Facebook, it automatically brings up your friend’s details as you type their name in “Send To”. And like Facebook, you can send messages to multiple friends.