Don't be evil
Every time I hear Google's "Don't be evil", I think of the Martians in the film Mars Attacks shouting "We come in peace!" as they engage in wholesale slaughter. An absurd correlation to an absurd movie, but "Don't be evil" is an absurd slogan for a company.
Facebook is at least as pernicious as it evolved from a couple dozen universities to the monstrosity it is today. People fail to realise that Google's and Facebook's customers are the advertisers, and the users are in fact the product they sell.