It will always be so. The power to control information is far too tempting to not be subverted for nefarious means.
It isn't just the social media giants. It applies to most if not all the corporate giants. Google's search and YouTube, Apple news, Netflix, Disney, etc...
replace "mainstream narrative" with "X narrative" and you have a point. 'mainstream' is meaningless.
At this point, by sheer institutional volume, 'mainstream' means 'American East Coast Left-liberal'.
I think i would restrict it to "X narrative, where X is chosen from spectrum of acceptable opinion dictated by appointed board of directors, USA, inc.".