I mostly agree with you. (I have 4 kids ages 9, 11, 13, and 23)
My Counter-Counterpoint is to first consider the following: (a) most kids are using normal social networks like Snapchat and Instagram in jr. high school (age 12+ in the US), (b) children see their parents and older kids using social networks and want to emulate them, (c) a lot of parents are either too busy or too lazy to do what you suggest and that's why so many children are handed a device at way too young an age, (d) helicopter parenting makes c. even worse.
So, given that, I say this should be built–if, for no other reason, than to encourage responsible and empathetic use of social media. Maybe it has time limits and is purposely non-addictive. Maybe it encourages IRL interactions at the park and helps parents coordinate that. Maybe it detects cruel comments and teaches them why its not ok. Maybe it guides kids through outdoor games and educational activities.
I like what you're suggesting but I'm more of the mindset "this is inevitable so i hope someone takes the moral/responsible route with it"
Also, a side note for those that don't have kids and aren't aware, a very large swath of elementary school children in the US already have Google accounts through their school or school district.
Edit: some words