LOL, this is just called getting older. As you get older, it gets harder to make new friends, harder to see your existing friends. I suspect it's always been this way.
They were hooked into the fabric of society enough that they contributed to social care, elder care, welfare, special needs and a whole selection of other roles that modern government have placed on the individual. Spun as giving "choice", the intent was to remove that bedrock foundation.
The removal of that foundation was as much part of 80s monetarism/neoliberalism as deregulation or privatisation. We gained individualism and private everything, and lost all the community resources that glued those individuals together.
Someone inevitably mentions Bowling Alone in these threads: https://en.wikipedia.org/wiki/Bowling_Alone
The real answer may be as simple as they got a bit older, people they know got busy with partners, family, kids, etc. and tend not to be available on random nights and weekends.
I'm willing to believe that there are meta-trends which have caused shifts in how and how much people socialize in addition to this but I'd need to be convinced.