As others have mentioned, the decline started long before social media and the Internet. When I was a child, in the 70s, my parents often had other families and couples over for dinner, and we often attended small dinner parties, too.
As an introvert, I absolutely loathed these gatherings and was frustrated by how powerless I was in avoiding them.
Recently, I have been reading over my mother's old letters she wrote to her parents at that time, in which she described the effort and social anxiety that went with this tradition. It was not something my parents enjoyed, but rather an obligation they felt they had to uphold. Someone invites you over, then that goes into an unspoken balance sheet, and you are required to reciprocate. She often spoke of "owing" dinner to people. I don't think she enjoyed it at all, though there were a few couples I can remember who were good friends of theirs, and those meetups were always a pleasure to listen in on. The rest were simply a drag.
I should note that their life in the 70s (in a middle-class college town) was not some idyllic paradise without the Internet, either. Everyone was constantly falling ill with colds or flu, and days were spent running dreary errands to shop for boring things that we now can get with a click. The rotary-dial telephone that hung in the kitchen was the heartbeat of the house, since replaced by Facebook and smartphones. My parents were always busy and exhausted.