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This is why you should block ads and trackers even if you're not a Facebook user. I have blocked all their domains in my hosts file also....but that's such a myopic view of the problem. Facebook has this data. There's an entire market, intentionally kept from the public, that specifically deals in the selling and trading of profile data. This is why the concept of "shadow profiles" is important.
Let's say I have three friends who are on Facebook and I am not (which I'm not). Those three friends upload my contact details to Facebook and, from these three data points, Facebook can derive certain assumptions:
Sex from my name.
General age-range based on my friends.
General occupation (let's say all three are co-workers or previous co-workers)
General interests based on their interests.
etc.
So, now Facebook has an ad-targeting profile of me. All they need to do to get more information on me, regardless of I'm blocking their specific domains, is to purchase correlating ad-profiles on what I'll call the advertising market.
So, my credit card purchases, my Google search history, etc. can all be correlated back down into this one shadow profile that Facebook holds on me and this can, in turn, be used to target ads to my friends. Not only that, but if I ever do sign-up for Facebook, they can immediately start targeting ads to me. (read: I'm not a net-advertising loss for being a "new user".)
I'm not saying that you shouldn't block Facebook domains but I think that, in the overall scheme of things, it's tossing a drop of water onto a raging bush fire.