I don't know quite in what sense Cuba is socialist. The pioneers of socialist thought (well before the Cuban revolution) described socialism as a moneyless society. Whether the state controls capital or private individuals do it is capitalism and is based on exploitation and minority power.

I highly recommend this article on Cuba.

You are correct to point out that capitalism is a nearly worldwide integrated economic system, and so must be socialism - a society of common ownership democratic control and free access to goods and services. Nations are part of the capitalist infrastructure. That is why socialist parties have formed the World Socialist Movement.