|I don't know how it got that way but sure, facebook has been blocked there so I reckon you are onto it.|
Have you got Norton on there (or did you have it in the past)?
Anything with # in front of it is disregarded, and simply acts as an inactive piece of text - so my question about Norton is purely out of interest.
If what you gave is the "complete end piece", it seems to me you can safely delete all of that. With Hosts changed to Hosts.txt when you open it you can simply edit it out in NotePad. Hosts might be Read-only so you would have to unhook that in Properties first. It is as well to make it Read-only after you have made the changes.
For your interest, anything starting with 127.0.0.1 (without the #) is blocked. Instead of going to Facebook you are diverting back to the systems local address (yourself) so not a lot happens.
If you still have it set to Hosts.txt you should find you get on to Facebook again, because changing the files name or extension makes it inactive (the system is looking for just plain Hosts).
How to know you are getting old 2:
Your feet seem further away from your hands