When did this start? IMHO it started with instagram. I remember back then there were multiple retro photo apps, insta was one of them, I had several on my phone and kept playing around with them (at the time apps felt like Christmas presents, each update exploring a device feature in creative ways).
I don't quite remember, but I don't think it was a social network then, I think you posted the photos in other networks, and then they made it into a social network and something strange happened. People started posting pictures of food and just general daily life stuff and I thought this was a small group of people who were a extroverts and just wanted to show off idk, they ate beautiful food.
Then something strange happened. This behavior started getting normalized, all other insta like apps disappeared and shortly after, it became necessary to have an instagram account.
I remember at the time I thought something was off, to this day I think I have posted a total of 10 instagram images, they still have the old filters, and stayed off of it since.
But it's been interesting watching it morph into this hydra that simply cannot be put down, to the point where it's more powerful than governments.
Really good question...TL;DR: I'd put it around when Mark Z decided Instagram also had to be Snapchat. (copied Stories) It normalized a behavior of copying.
I had gotten completely out of these apps, then ended up in a situation where I needed to use Snapchat daily if not hourly for messaging, and needed to use TikTok to be culturally literate. (i.e. I got into something romantic with someone younger).
It was a stunning experience. Seeing _everyone_ had normalized this "copy our competitor" strat, hill-climbing on duration of engagement.
YouTube Shorts is a crappy copy of TikTok with mostly TikTok reposts and no sense of community.
Snapchat has a poor clone of TikTok that I doubt anyone knows exists.
TikTok is the ur-engagement king. Pure dopamine, just keep swiping until something catches your attention, and swipe as soon as it stops. No meaningful 1:1 communicating aspect (there's messages, but AFAICT from light quizzing of Gen Zers, it's not used for actual communication)
Instagram specifically is hard for me to speak to, because Gen Zers seem to think its roughly as cool as Facebook, but my understanding is millennials my age or younger (I'm 37) use it more regularly, whereas Gen Z uses it more as like we'd think of Facebook, a generic safe place where grandma can see your graduation photos, as opposed to spontaneous thirst traps.
it started with FB before IG, but infected IG once Zuck bought it
but things really took off when TT cracked the code for endless scrolling of "relevant" content