The first aspect is herd behavior. I used to take my kids to after school academics (Kumon, although it doesn't matter) and the waiting room was fascinating as the tiger moms (and dads) universally whipped out kindles and the au pairs whipped out smartphones and texted. Strict social classes by device selection. As publicly as possible, strivers read, servants text. As a parent I felt the need to kindle along with them, as its not like anyone socialized and that's what the parent class shows off in after school academics waiting rooms. Possibly no one on the subway wants to social media, but everyone on the subway social medias, so they feel the herd instinct... I worked at a building once with a large poor population (call center on some floor or another) and its a poor person fad to do pedestrian stuff like walking around parking lot while speakerphone talking or preferably video conferencing, like everyone under $20K/yr did that for awhile and they had to copy each other, you'd have like 10 people come off shift and all of them talking to their phones not each other.
The second aspect is social assumption. All smartphone use in a car is labeled as "texting" although I haven't used legacy SMS messages since about the time I got a smartphone (although SMS texting long predates smartphones of course). Culturally among non-technical people all messaging apps from Slack to IRC to the big advertising companies is "texting" which leads to confusion when a normie asks you to text them, they mean guess which of the dozen or so popular messaging apps you connect with them on, whereas I'm trying to remember how to use old fashioned SMS on my phone. Likewise everyone on the subway does social media, even if everyone is actually changing podcasts/music/audio books or reading ebooks or general web surfing around or whatever rando thing. I feel guilty about the endless indoctrination on billboards about "texting kills" when driving, especially when I'm merely changing my audiobook in audible while at a stop sign, not texting someone a copy of War and Peace at 85 MPH, although I don't feel any discomfort about the endless indoctrination about "drinking and driving kills" despite sipping tap water from a water bottle occasionally in public while driving. Worry of what if someone thinks my water bottle is full of grain alcohol isn't part of culture as much as what if someone thinks my safe audible app is (literally) killer app facebook messenger?
Third aspect as an older guy I can still remember when the default "I'm bored" activity was mindless TV watching. You might wish the default bored activity is not social media but something more useful, like online MOOC education or reading great books or prayer or whatever but lets be realistic, endless scrolling social media is of the Mork and Mindy syndicated TV show rerun social class of activity, the people doing it will merely find something even more inane to fill downtime, perhaps spinning fidget spinners to ever higher RPMs. As an inane time filler, social media isn't that bad compared to the realistic practical alternatives.