Be sure to watch through to the end for the conclusion! I'll summarise: Click farms like every page they can, far more than just the pages they were paid to like; the theory is that they will evade detection this way. Paying for "legitimate" facebook exposure will expose you to the click farms as well, and that will be the absolute majority of your gained likes. In this way facebook ads give a huge, but useless, gain…
Does that matter? I mean, obviously if you're trying to work out whether it's worth paying for Facebook ads, it's worth being aware that a large number of Likes isn't necessarily a sign of a successful ad, but you don't know the "value" of a Like in advance anyway. If you discover that every 1,000 Likes you get 10 more sales (or whatever), then you can work out whether it's worth spending money for an ad that will likely bring in 1,000 Likes, whether 990 of those likes are "genuine" Likes that don't turn into sales, or 990 of the Likes are click farm Likes.
I mean, I've never advertised on Facebook, I don't know anything about it, but I would have thought you need to measure the actual effectiveness of your ads; whether click farm fake Likes are contributing to this overall effectiveness or not doesn't seem relevant to the end result.