Why would Yay get adopted as "dislike"? Something like angry or sad would make way more sense. It'd be incredibly hard to create this culture in 1.4B people when the UI indicates otherwise. Perhaps among younger audiences? I'm still very skeptical.
I associate and would use Yay only as an ironic expressions of excitement, as in "Two more weeks of working overtime, yay", but I have only found a discussion in an catholic forum as evidence that some people think the same way.
I don't believe I have ever heard someone use "Yay" in a non ironic fashion, but I'm not from an english speaking country.
So you could easily imagine a situation where posts by popular kids would get likes but posts by outsiders only yays.
English-speaker here (UK) - just to let you know that Yay! is very definitely used in a non-ironic fashion all the time as a light-hearted, perhaps slightly jokey, expression of delight. In text, and sometimes even in speech.
I can 'calibrate' a Yay to indicate a negative sentiment based on context, but it's a bit jarring and not even very strongly negative. Examples like "Two more weeks of working overtime. Yay" is obviously a negative use but, to me, that's you indicating disapproval of a fairly trivial inconvenience or annoyance in a rather light-hearted way. If you really disliked something I'd expect a more direct statement like 'damn it' or your whatever your chosen flavour of profanity.
The idea of Yay as a general meme for negativity is very jarring, at least to this native reader, and I'd really struggle to imagine it ever catching on as a result. Hope you found this perspective useful!