The free app does not require a Vonage subscription, but does require both parties to have the Vonage Mobile for Facebook app installed. Once installed, calls can be made over 3G or Wi-Fi.
The app is pretty simple to set up and only requires that you enter in your Facebook credentials once. Friends are displayed in different groups; those that can be called with the app have a "V" logo next to their names. If a friend is online but doesn't have the app installed, you can chat with him or her and also send an invite to download the app.
Like the most recent Skype Mobile release, the app can run in the background, meaning that even if it's not open, incoming calls can still come through. When an incoming call comes through the app, the caller's Facebook photo and most recent status update are displayed on the screen.
This video shows how the app works:
The concept of making voice calls over Wi-Fi or 3G using a VOiP mobile application may no longer be new or novel, however, we appreciate that Vonage is targeting a specific set of users. For many individuals, Facebook is the nerve center for all online communication -- building a VOiP app off of Facebook is pretty smart.
An iPad version of Vonage Mobile for Facebook is poised to join the iPhone, iPod touch and Android versions soon. We hope that Vonage will also consider bringing the app to BlackBerry handsets.