This isn't the first appearance of voice calling in Facebook for Android. The original release was early last month for Canadian users. But, given that the US is about 10x more populous than Canada, that release can basically be considered a beta test. Facebook claims that there are actually 24 countries where the Android version of Messenger offers voice calling, but we haven't seen those reports before.
The update today is only for US users, and it isn't the main Facebook app, but the Facebook Messenger app. Once you've updated, you will get options to make voice calls to your friends who also have the VoIP enabled Facebook apps. Inside the app there will be a small "i" in your conversation thread that allows you to start a call, or in Chat Heads, the option is hidden in the vertical ellipsis overflow menu.
Facebook reps told The Verge that so far VoIP calling has only been a "blip" on the company's servers, so it probably shouldn't be too long before we see the full global rollout.