What converting to MP3 actually does
A Facebook video is a container holding a video track and an audio track side by side. Converting to MP3 takes the audio track and re-encodes it on its own, discarding the picture entirely. That is why the result is dramatically smaller — usually a few megabytes instead of tens or hundreds — and why it takes a moment: the audio is genuinely being processed, not simply renamed.
Bitrate: why a higher number is not always better
Facebook stores audio as AAC, commonly somewhere around 128 kbps. Converting that to a 320 kbps MP3 does not add anything back; it produces a larger file containing exactly the same sound, because detail discarded during the original encode is gone for good. The honest ceiling for any conversion is the quality of the source.
In practice 128-192 kbps is the sweet spot for material that came off Facebook. For speech — interviews, talks, podcasts — even 96 kbps is usually indistinguishable, and the file is small enough to keep hundreds on a phone.
When audio-only is the better choice
Extracting the audio makes obvious sense for anything you listen to rather than watch: recorded talks and conference sessions, interviews and podcast episodes posted as video, sermons and lectures, language practice you want on repeat, or a set you would like on your phone without a video file draining the battery. Audio also plays with the screen off on every phone, which video generally will not.
If you want the picture as well, use the video downloader instead and keep the MP4 — converting afterwards only costs you a second pass of quality loss.
Videos with no audio to extract
Some videos genuinely have no audio track: silent screen recordings, captioned clips built for autoplay in a muted feed, and time-lapses are all common. If a video has nothing to extract, the MP3 option will not be offered, which is deliberate — a silent file is not worth the wait.
Music, podcasts and what you may keep
Extracting audio does not change who owns it. A track posted to Facebook is still the rights holder's work, and a licence that lets it play inside Facebook's apps does not extend to a file on your computer, to a video you upload elsewhere, or to anything commercial. Keeping a talk to listen to on a commute is ordinary personal use. Building a music library out of other people's uploads, or putting extracted audio behind your own name, is not — and automated systems catch it reliably.