If Free Basics is not supported in your country, downloading it via the APK is the one and the only option that lets you use Facebook, as well as other, basic websites for free.
To get the app, download Free Basics APK file from any of the third-party websites and head to device settings.
Then, visit Apps and Permissions > Manage Apps. From the list of applications, select your browser and enable the option Install Apps from Unknown Sources. To install the Free Basics app, click on the downloaded APK file and hit Install.