>n fact, I'd argue that mainstream social media can never be countercultural.

I would say that is pretty much true by definition. Online countercultural elements are open source software and other collaborative platforms like Bandcamp and similar sites where people are trading tracks and what they have created. Torrenting is still countercultural. What the counterculture has in common now is that while the individualism is still present in the creation of music/software and the political/social choice to collaborate, the collective action of collaboration is the focus of attention, not individual fashion and identity. That kind of individualism has been sold back to us ad nauseum and its apotheosis is Instagram.