Most arguments about “net neutrality” neglect an important reality: The internet most of us use is already far from neutral, thanks to the profit-focused algorithms and opaque content guidelines by which social-media companies such as Facebook, Twitter and Instagram govern their sites.

Through these personalized online portals, we read news stories, share videos, learn about our friends’ lives, and discuss social trends. But these platforms don’t treat all content equally or distribute it fairly. Facebook and Instagram manage...