One of features promised with the Facebook Messenger for iPhone update a few weeks ago was Voice-Over IP calling, so that you could use the app to call your friends over a mobile or Wi-Fi data connection instead of your plan's minutes. The feature is finally rolling out to phones in the US, with the UK hopefully to follow shortly.
We tested voice calling over our office Wi-Fi in the US, and over straight data, and it worked perfectly. Hell, we even called Andrew L up in Toronto from New York without a hitch.
To use the new VOIP, click on one of your friend's profiles as if to initiate a message, then click the little information button in the top right corner, and then from the following screen click "Free Call." But remember, while the calls don't use your plan minutes, they're not necessarily "free." If you're not on Wi-Fi, regular data usage rates apply.
But what's pretty awesome is that you don't need a data plan, minutes, or even a phone at all. As long as you're running a device with iOS 4.3 or higher — an iPad or iPod touch for example — you can call over Wi-Fi, just like Skype! [iTunes]