What the feed changed, and it was not the habit

The habit is old. What changed is who you are measuring against.

For most of human history the people in your sample were the people on your street, at your work, in your family, at your masjid. A few dozen of them. You saw them on ordinary days, in ordinary clothes, having ordinary arguments in the car park.

Now the sample has no limit, it has been sorted so that the people who show up are the ones doing well at the exact thing you are anxious about, and you are seeing them on their best hour. Not their best day. The best hour of their year, chosen by them, and then edited.

That the sample is what matters is not a modern discovery. Al-Ghazali sets it out in the Ihya, nine hundred years before the 1954 paper, and gives the same reason: people are set against each other by proximity and by shared aims. He states the negative case exactly, إذ لا رابطة بين شخصين في بلدتين متنائيتين فلا يكون بينهما محاسدة, our rendering, there is no tie between two people in two distant towns, so there is nothing between them. And ولذلك ترى العالم يحسد العالم دون العابد, our rendering, this is why you see the scholar envying the scholar and not the worshipper. The cobbler and the cobbler. A man and his cousin, more than a man and a stranger.

Two things about borrowing that. His word there is hasad, and a few steps from now this lesson will say plainly that what you feel is almost certainly not hasad, so nothing in this paragraph is a description of you. What is being borrowed is only the structural half. Comparison needs proximity and a shared aim, and the two distant towns were the reason most people were never in your sample. A feed abolished the distance. That is the whole change.