The Facebook Beta app for Windows Phone is an app developed by Microsoft to support the Facebook social network on mobile devices. The app is made in-house by Microsoft but also in collaboration with Facebook themselves for more technical issues involving the site and service.
Facebook beta as the name implies is the test version of the official Facebook App for Windows Phone. Microsoft opened up access to anyone who wants to receive early access to new features coming to Facebook for Windows Phone, and also as a response due to the negative feedback (and poor ratings) of the official app.
The Facebook App and Facebook Beta App are developed in conjunction with the beta often having new, unreleased features that trickle down to the non-beta app after thorough testing. The beta is open, meaning there is no registration nor is feedback required, although users can submit comments on the app. Likewise, Microsoft has a User Voice forum for discussion and feature requests from consumers. That forum is found here wpfacebook.uservoice.com.
The first beta version of Facebook arrived on April 30 2013. The app is updated on a regular basis every few weeks, often with just minor tweaks and bug fixes, though occasionally a significant feature or redesign gets released. In June 2014, the Facebook beta app received a major UI overhaul and several new features. In July 2014, the beta app picked up Facebook Messenger support by tying into the new standalone app.
Currently, the Facebook Beta app contains some Windows Phone 8.1 features not found in the regular version. As such, there are two different builds denoted by 8.1.x or 5.1.x, though they are the same app.
Users can download the Facebook Beta app for Windows Phone here or by scanning the QR code in the upper corner.