By definition, this is "absence or suppression of passion, emotion, or excitement." When adding the social aspect to this, it creates a negative connotation. Not only is one absent of their own life's situations, they care even less about the situations of others-- even if they can do something about it. Social apathy is a desensitization of humane involvement where society is concerned.
How it works
The belief that "you can't fight city hall" helps to ensure inaction. Basing one's know of historically failed attempts, where there is a small group concern for important issues, they can "pass the buck" and ignore the problem.
Much like "Unacknowledged Privileges" this a great pattern of avoiding opportunistic callings. Whenever one comes upon a chance to make a difference in another person's life they can fall back on the fact that it is "not their problem."
The events of Pearl Harbor rallied America to arms in joining WW2.
See: US Non-Interventionism Policy:
George Washington's farewell address is often cited as laying the foundation for a tradition of American non-interventionism:
The great rule of conduct for us, in regard to domestic nations, is in extending our commercial relations, to have with them as little political connection as possible. Europe has a set of primary interests, which to us have none, or a very remote relation. Hence she must be engaged in frequent controversies the causes of which are essentially foreign to our concerns. Hence, therefore, it must be unwise in us to implicate ourselves, by artificial ties, in the ordinary vicissitudes of her politics, or the ordinary combinations and collisions of her friendships or enmities.(1)
By not affiliating too closely with the rest of the world, the US attempted to avoid involvement in a situation that affected everyone. This elevates Social to National Apathy. The US were pivotal in tipping the balance of WW2, even though their hand was forced.