So it's easier to have 2~4 aggregators in where all the information you desire resides, even if in each of them there are different forums.
A unified entry point helps adoption.
Read a cool blog post? Nobody around you will ever give a shit, because in order to do so, they'd have to read it too. Shared a photo from a vacation? It might start a conversation or two with people around you, while you receive dozens or hundreds of affirmations (in the form of likes).
I don't like to use social networks, but that's what I fall back on when I have a few minutes to spare. I rarely look at my list of articles I've saved for later — who has time for that?
Plenty of people. Ever push an article to a reader view service and see how long it takes to read? Most articles posted here on HN or the nyt front page can be read in 3-5 mins. Occasionally you'd get a 20 min slog.
I used to use social media way more, and by far my biggest wastes of time on the platform were those spare minutes you get a dozen times a day. On the elevator, waiting for the bus, waiting on food, anytime I could sit still the phone went out and my head went down because that's what everyone around me was also doing while waiting on their coffee.
Eventually I realized I was just idly scrolling and not retaining anything at all from those 30s-2m sessions on instagram. Just chomping visual popcorn. Now, anytime I have a spare 10 mins, I'll read an article or two from my reading list. Anytime I have less than a spare 10 mins, I'll twiddle my thumbs and keep the phone in the pocket.
I used to be much more scatterbrained and had trouble winding down for the evening and getting good rest. Now, I feel like a monk.
b) mainstream culture > closely-knit communities (facebook > forums)
c) big-player takeovers (facebook for groups, google for search) over previously somewhat niche areas and, actually, internet infrastructure
d) if you're not a big player, you don't exist... and back to c)
You chose Instagram as your example, to make the point that phones favor consumption over creation?