Vonage has introduced two apps that allow you to make free phone calls to your Facebook friends nationally and internationally.

The apps, called Vonage Mobile for Facebook, are available free for the iPhone and Android phones.

The app is very simple to use. Load it and sign in as you do for Facebook. You will be asked what information you want to allow your friends access to, and you will probably at this point receive the message “Less than 5 of your friends have Vonage mobile for Facebook.” It then suggests that you invite some.

If you do have friends who are members, you can click their names and be connected.

Calls can be connected over Wi-Fi or over 3G, although using 3G will use your data minutes. Connections made over Wi-Fi are free – and you save voice minutes even on local calls.

Friends who are online for Facebook chats will show in your contacts list with a blue icon. People who are not online have a gray icon (although in a test on an Android, the phone and my Facebook home page didn’t agree on who was available).

You’re probably wondering what Vonage gets out of this. The company says it may add other pay services to the app, like low-priced text messaging, and it may help them get into new markets.

For now, you get free phone calls.