It started convincingly in the 60s (tracking on Bowling Alone, a book written in 1999 about the decline of the American community), but isn't clear-cut on likely reasoning at the about halfway point I'm at, other than older more socially-involved generations dying out.

My take expanding what I've learned so far is that appears to be a combination of:

• there are things that are 'entertaining enough' at home to discourage former events like going over a friend's or coworker's, so general schmoozing is relegated at best to video games online with rare exceptions. The 60s is when television really "took over" the family room, but there was radio and books before it.
• the generation that grouped together to fight the oil barons died out, and did not transmit their union dinners and other community behaviors to their children.
• churches saw significant decline during the social movements of the 60s that has continued since.

So we may be too comfortable at home with new technology to go out, even if we're not fulfilling all of our social needs (that is, the "epidemic of loneliness" so often brought up). And when nearly all the interactions you have left is "vote with your dollars," whom you choose to pay becomes the political and social choice that defines who you are.*

* though the fact that a few people own most of the dollars means your vote basically means squat anyway, but that's getting on to class war, for which social isolation is a fantastic benefit for the ownership class against labor negotiation