Here are the steps to download Facebook videos on your iPhone:

1. Open the Facebook app and find the video you want to download.
2. Tap on the video to open it.
3. Tap on the "Share" button below the video.
4. Tap on "Copy Link".
5. Open your mobile browser and go to fbdown.net.
6. Paste the link you copied into the box in the middle of the page.
7. Tap on "Download".
8. Tap on "Download Video".
9. Choose "Save Video".

Alternatively, you can use an app called MyMedia to download Facebook videos on your iPhone. Here are the steps:

1. Open the Facebook app and find the video you want to download.
2. Tap on the video to open it.
3. Tap on the "Share" button below the video.
4. Tap on "Copy Link".
5. Download MyMedia from the App Store.
6. Open MyMedia and go to fbdown.net.
7. Paste the link you copied into the box in the middle of the page.
8. Tap on "Download".
9. Tap on "Download Video".
10. Choose "Save Video".