I blame ubiquitous access to cars and phones as significant early contributors to this problem, with modern IMs and social media being the final nail in the coffin. Because together, these technologies enable your social network to be mostly independent of location. Nowadays, when you move to a different neighbourhood or a different town, you'll still be in daily contact with your friends and family from the previous location, and (if they're relatively close by car) you'll also hang out with them regularly. So you have no reason to try and meet new people at the place you now live in, because you still have your old social network! Hell, new friends will be competing for attention with your old friends - where just few decades ago, these old friendships would go to "hibernation mode" by virtue of distance.

Internet communication, with its much richer modes of communication, is essentially social junk food. It's not a real substitute for meatspace interactions, but it satiates the hunger for socialization. Thus people feel ever more connected and ever more lonely at the same time - loneliness being social malnourishment.