Since the 1970s, the General Social Survey has had questions about "how often you spend a social evening with" various kinds of people: "someone who lives in your neighborhood," "friends who live outside the neighborhood," and "relatives." The questions are on a seven-point scale going from never to almost every day, and I recoded them so that higher means more often. Because people often say that the negative trend is mostly among the "working class" I calculated the average separately for people with and without college degrees (the blue line is without and the red line is with a degree). The means for spending an evening with neighbors:
A downward trend for both groups--maybe there is something to the idea of an epidemic of loneliness?
Friends outside the neighborhood: maybe a slight downward trend for people with college degrees, but a more definite upward trend for people without college degrees. Since there are more people without degrees than with degrees, the trend for the population as a whole is slightly upward.
Relatives: a pretty clear upward trend for both.
So in addition to the lack of direct evidence of an epidemic of loneliness, there is evidence against the idea of a general decline in social contact.
Loneliness is a real social problem, and claims of an epidemic seem to be more effective in getting people to pay attention than just saying that something is a problem and it hasn't been getting enough attention. So in that sense, claims that loneliness is rising don't do any harm and may do some good. However, they are often offered as an explanation of rising political polarization (as in Sasse's book). In this respect, they are harmful because they divert attention away from the real causes of polarization, which I think involve political and media elites, especially on the conservative side.