We render out mashes using Apple's H.264 codec at 1080p. Which is what Facebook recommends. There is a setting for the iOS camera that allows you to record at 720p or at a higher frame rate that turns on 720p automatically.
To ensure your upload is of the highest quality make sure in the settings of Facebook, under Video Settings, "Upload HD" is selected. Steps for this are here.
Check to make sure that in your iOS Facebook settings, you have "Upload HD" turned on (it is off by default).
When you initially upload a video, it takes time for FB to post the high res version of your video (especially if it is larger in size)- it's basically processing in the background. FB will give you a lower resolution version to view in the meantime, making it seem like your video has not uploaded properly
FB could be throttling the size of your video based off of size, internet connection or any number of other fun FB factors.
Sometimes within FB, with a larger video, you need to press the tiny "HD" watermark on the video to actually see it in HD.
You can also find this setting in your iPhone's main Settings > Facebook.