> The sharp rise in both polarization and the number of close friends occurred between 2008 and 2010—precisely when social media platforms and smartphones first achieved widespread adoption. This technological shift may have fundamentally changed how people connect with each other, indirectly promoting polarization.

Indirectly? Seems to me that this is far more likely the "direct" cause, given what we know about the psychology around algorithmic feeds.

Also - I'm not sure if I missed it in the article, but did they define what they mean by "close relationship" means? I'd be very curious to know if a purely online relationship is counted and how this may also contribute to the observations made.