as mentioned above, or additional:
Things you might be doing wrong;
filming in low quality
converting your videofiles and losing resolution in the process
Exporting files in the wrong format/resolution(experiment with what works best, i usually export my gopro HD footage in 720p wmv files and that works best in both quality and size of the files....)
facebook doesn't display full HD videos
look into something like vimeo for better performance
And right after uploading your video might look like crap because the servers are still processing it ....