What I find most interesting is the very clear but often ignored 'Cloaking Guidelines' by google:

"Cloaking refers to the practice of presenting different content or URLs to human users and search engines. Cloaking is considered a violation of Google’s Webmaster Guidelines because it provides our users with different results than they expected.

Some examples of cloaking include:

...

- Inserting text or keywords into a page only when the User-agent requesting the page is a search engine, not a human visitor

" [0]

Google is happily showing LinkedIn, FB, pinterest and news sites content. But when I, Joe User, go to the page, I see nothing but some login/register/pay now form. How is this not a violation of the cloaking guidelines? Clearly google is getting different content than what I am!

(Presumably this is how article's extension works... by masquerading as GoogleBot -- again proving that these sites are serving up different content)

[0] https://support.google.com/webmasters/answer/66355?hl=en

edit: formatting