The struggle for power which brands are destined to lose
Relevancy and purpose versus yield
Advertising works best within a context where customers perceive it to be useful, which is one reason why Google's advertising yield is ten times that of Facebook (customers who are searching are more likely to welcome a promoted solution).
The clue is in the title - 'Social'
There's plenty of evidence about the science of sharing, recognising the virtue of stimulating people's interest through strong positive emotions.