You'll need to play the video with a program that offers a 'snapshot' function and use that to obtain a 'still' from the video. (Note that the quality from such a still is often less than you'd expect. Videos are made up of thousands of frames that merge together to look good when played as a whole but which often don't have as much detail in an individual frame as a proper photo would).
Windows Media Player doesn't have a 'snapshot' facility. (It's technically possible to extract a single frame using Windows Media Player but it's a complicated process). Download and install VLC to your computer:
http://www.videolan.org/vlc/index.en_GB.html
(It's about a zillion times better than Windows Media Player anyway, so it deserves a place on your computer).
Transfer the video file from your camera to your computer and then play it using VLC. Press Shift and S together at the point where you want to take a snapshot from your video. (You can do it as many times as you like. The snapshots will go into your 'My Pictures' folder). Then upload your chosen snapshot to Facebook.