This quote sounds very much like something that Robert Putnam might have said in his book Bowling Alone, published in 2000. Putnam argues that Americans are participating less in various social organizations and are, therefore, feeling much less social cohesion and solidarity. This is the general meaning of the quote that you cite in this question.
First, the quote argues that people are participating less in institutions. These are things like churches or fraternal organizations. They even include bowling leagues, the lack of participation in which inspired Putnam’s book title. People no longer want to participate in such things. Instead, they prefer to stay at home and entertain themselves using things like television or the internet.
The second part of the quote argues that this change makes people feel isolated and lonely. In the past, Americans belonged to more organizations, both formal and informal. These institutions created large numbers of social ties for them. If they needed emotional support, for example, they could typically do things like turning to their churches or the people they knew from their labor union. They could turn to people in their fraternal lodges. Today, people lack such support. They do not feel like they are connected to many people. They feel that they are alone in society. This is what the quote means by talking about “isolation” and a loss of a ”sense of belonging.”