From what I've observed, the social media reflect that what is best and also what is worst in cultures. If you think that society is becoming dumber, it is not the fault of social media.
Social media was used to overthrow several corrupt authoritarian regimes and used in Egypt to educate and inform, for instance. In a culture determined to bring about greater justice, social media will be used in constructive ways. In cultures more interested in amusement, it will tend to be used in trivial ways.
The type of education system that has evolved in the US will ensure an increasingly dumber society without needing the aid of social media.