All of your rendered and completed videos can be found in your Lumen5 Dashboard. Clicking on any of the video previews in your dashboard will take you to the video page.
From here you can click to share to Facebook. You can only share to Facebook Business Pages directly from Lumen5.
To share to a Facebook personal profile or group, click to download the video. Once you have downloaded the video file, you can post it onto Facebook the same way you post photos.