Why would so many sites do this for Facebook's benefit?
The sole purpose is for advertising, which is how they make money. The thumb will have the article show up to your friends, and people who have similar interests to you. The pixel also helps with advertising, because they build a picture of who you are and can give you relevant ads.
Well, I personally pay for a guardian subscription, but block ads. This is because I consider ads a net-negative in my life: they actively lower the quality of pretty much everything around me, including the content pimping the ads.
Meanwhile, a facebook thumb button is net-negative only if the guardian doesn't get paid--it essentially becomes a transaction where I pay facebook with information and then facebook pays the guardian with money.
If this assumption is false, I'm happy to continue blocking the passive pixel and thumb trackers and continue to support the guardian directly.
Why would so many sites do this for Facebook's benefit?
The sole purpose is for advertising, which is how they make money. The thumb will have the article show up to your friends, and people who have similar interests to you. The pixel also helps with advertising, because they build a picture of who you are and can give you relevant ads.