The facebook SMS feature allows the user to choose the messenger app as their preferred SMS app. You will be able to send and receive texts, videos, images and audio including rich contents such as stickers and emojis.
Other form of content like GIFs, video and calls etc would have to be communicated using the messenger instead.
Facebook assures you that your privacy is safe as it doesn’t send, upload or store any of your conversations on its servers as all messages are sent and received via standard SMS protocol and not cellular data.
If you’d like to use the messenger app as your default SMS app on Android, simply make sure you are running the latest version of messenger. Thereafter, launch the app and tap on settings > select SMS and turn it on as the default SMS app.