Facebook has just invited me to interview and now I’m pissed off

This seems like a very childish response to me. Perhaps it would have been better to simply decline the offer and list your reasons, rather than berating the recruiter.

As it currently appears, the argument is not even very clearly thought out. You just kind of yell at them about slavery and feudalism, which imho doesn't have any relation to the puzzle you were asked to solve.

I agree that I wouldn't have done it, but then again, I don't want to work for Facebook.

Slavery, Factory Sweatshops?

You have a chance of a job at a household name company (who can afford to be selective) , probably with cushy salary and benefits.

8-10 hours worth of work isn't a great deal, especially if they give you a few weeks. Even a person working a full time job should be able to squeeze that in by temporarily reducing hours spent watching TV.

It's not like facebook even get any benefit from you performing this work for them.

sounds like a Great Filter that author's ego didn't fit through (not that i'm in anyway critical of the author, as until we're back in 2003 labor market or i start to feel [undesired] impact of my age on my opportunities, my ego doesn't feel like trying to squeeze through even coarser filters), it is just that moral/ethical/emotional appeals (slavery/etc..) don't work in market.

I don't get it. What benefit are they deriving from the puzzle being completed? It doesn't even compare to regular work in that way, let alone sweatshops, assembly lines, whatever.

A lot of people would go to a lot further lengths to work at a place like Facebook, that's for sure. The holier-than-thou attitude here is so aggravating, for many reasons.