“One is the loneliest number that you’ll ever do.” So sang Three Dog Night in 1969 in their hit song “One.” Six years later, a best-selling item for a while was the “pet rock,” a stone taken home and treated like a pet. Yes, that’s right, people actually bought rocks and either kept them for themselves or gave them to others as gifts, companions they could talk to without all the fuss and mess of a dog or a cat. Both the song and the pet rock craze reveal that feeling lonely isn’t some new American phenomenon. In the early 21st century, social media promised new avenues to friendship. MySpace, Facebook, and similar sites attracted millions of users, then hundreds of millions. You could communicate online with “friends” and followers, sharing your life, your successes (rarely your failures or your interior miseries), and personal news, again nearly always positive, about …