What is the main idea of Bowling Alone?

The main idea of Bowling Alone is the decline of social capital in the United States over the last half-century or so. Social capital is a sociological term that refers to those factors of effectively functioning social groups such as shared values, mutual trust, and reciprocity.
As a result of this decline, Putnam argues, Americans have become more alienated from each other and less likely to engage with one another through civic groups and organizations. In the political sphere, the decline in social capital identified by Putnam has led to a growing lack of involvement in the democratic process, fed by a profound distrust of politicians and the whole political system that has accelerated since the 1960s.
Putnam illustrates his main idea through the example of bowling. He notes that although the number of people who bowl in America has increased over the years, the number of those who bowl in leagues has declined. If people choose to bowl alone, this inevitably means that they're effectively cutting themselves off from any meaningful social interaction. This is a metaphor for the decline in social capital that has taken place in the United States over the past few decades.