tl;dr: technology in the late 90s and mid 00s gave power to users to be more social on their own terms, technology in the late 00s took power away from users and gave it to platforms to dictate social interaction with notifications

It appears that this dip in loneliness started as early as the late 90s when internet and cell phone technology was connecting us and enabling us to be more, deliberately, social. Perhaps in the late 90s to mid 00s we found the sweet spot for balance between social technology and loneliness. After the mid 2000s, Facebook became a thing, smart phones became ubiquitous, and user behavior was increasingly driven by non-deliberate means: notifications.

Notifications are making people miserable because its turned social into a fucking chore. You are less likely to have the social engagement you decided upon, and instead to have one generated for you by an algorithm.