A gain screenshot carries almost no information. It shows one position at one moment, with no account size, no starting capital, no other positions, and no history of what it took to get there. The same account can produce a spectacular screenshot and a terrible year.
The harm is distributional. Feeds show the extreme tail of thousands of accounts, so the visible sample makes enormous returns look routine and makes a sensible year look like failure. That warped baseline drives oversizing more reliably than any single bad decision. See survivorship-bias-in-advice.
Where losses are posted too, the culture is at least honest about variance - see loss-porn. Where only gains appear, treat the feed as advertising.
Related: loss-porn, survivorship-bias-in-advice, comparison-trap, finfluencer