There are many ways in which to answer this question, as there are many different themes in the book. In light of this, I will give you a few ways in which the book reflects modern society.
First, Golding plays with the ideas of civilisation and savagery. He tries to show that all people have differing degrees of both. He also suggests that the impulse towards savagery is more basic in people and that society holds people in check. This is a profound point, as we can see this dynamic in our society in many ways. Think of all the protest of last year in the Middle East and the Occupy Wall Street.
Second, Golding also plays with the the ideas of morality and immorality. There is a fine line in this area as well. This, too, is an important point, because we see that both morality and immorality are part of all people.