It's simple supply and demand. Facebook no doubt have hundreds of applicants beating down their door every single day.

Some of these people will be great, some will be ok and some will be useless.

Throwing up a filter like this weeds out people who have no chance of a successful interview or those who don't want the job all that badly.

You would seem to fall into the latter camp.

8 - 10 hours for a prestigious job like FB isn't all that much. I have heard of people doing weeks of unpaid work for a chance at a minimum wage job.