what i'd like to know is... how can a full node, reading the blockchain, and trying to validate it, possibly tell whether all the accounts claimed to be spun off out of Facebook or Reddit really were spun off out of Facebook or Reddit?!

I mean, that's something that depends on proprietary data of those two companies!

can someone in possession of the true, raw blockchain, look inside it and see how F&R-spun-off accounts are represented? what's to stop a miner just inventing dozens of them that aren't really linked to F&R at all?