If we all became non-selfish, would the American economy shut down? :)?

Being altruistic (non-selfish) is perfectly compatible with a private free market economy. Nothing stops anyone in such an economy from giving away some of their income to others. Workers could give more to the poor. Company owners could raise wages above that required by the market or sell products at below the cost of production.

However, it is dangerous to adopt an economic system such as communism that is based on the assumption that everyone will be altruistic. Remember the famous saying of Marx about a true communist society -- From everyone according to his worth, to everyone according to his need. Everyone works hard to produce for the common good and is willing to accept an income based on his need rather than his productivity.

However, if you are skeptical that most people will not be altruistic, then stick with a private market economy. The basic idea going back to Adam Smith in 1776 is that even though everyone is only concerned about their own self interest, a competitive economy will channel this selfishness for the benefit of the entire society.

Of course the economy wouldn't shut down. We would shift to what's called a gift economy. People would produce all the same stuff, but instead of doing it for their own gain and the gain of their families, they would do it for everyone.

I'm a free market advocate, but the way you phrased your question really doesn't have any other answer.

No, I think it would thrive and we would thrive socially as well. Right now, we have mega corporations that own all of the pieces of the puzzle because they want to make the most money possible. If we weren't selfish, everyone would have their own personal piece to contribute and we would all make money. Capitalism at it's finest.

No, it would shut down because people would be satisfied with themselves and we would all do as little work as possible and perhaps share inconvenient tasks. That is the 'Fable of the Bees'.

Hmmm... Probably, if not shut down completely, suffer a huge depression, leading to massive famines and poverty rates.

No because we still all have to live. It would certainly be different however.