Adding to this, text messaging and emailing and deliberate and specific, which means someone has to think to include you. It takes work to figure all that out--it is an active decision to communicate instead of a passive broadcast about something general that invites people to participate. It's very easy, and a good way to keep relationships warm and to make room for serendipity.
I don't know about you, but I really dislike things like personal newsletters and text messages/emails just to strike up any conversation, probably because at any given time I'm probably pretty busy, and being present requires both thought and time. On the other hand, when I share something on social media, it means I'm pretty much willing to engage with all comers, or if I see you've updated on social media, I'll assume the same.
Secondly, if you're that random friend that insists on only communicating via text message or email or some weird idiosyncratic way that applies only to you, I'm probably going to leave you out of a lot of stuff, even if I like you, because I have limited brainpower and I'm going to forget when I'm managing 15-20 people on a social channel and trying to update you out in the wilderness.
Lastly, email and text really, really suck for discussing topics in large groups, so if I send a link I want to talk about to a bunch of friends, I have the choice of managing 15 individual conversations (with lots of repetition), or managing an incredible amount of chaos.
So yeah, IMHO social media has definitely solidified a place in the communication stack.