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I'm just guessing but their's probably some cat and mouse involved. If facebook starts filtering one social engineering vector then they'll just shift to another one. It's like banning bots in various video games, you do it in random intervals in waves that obfuscate the exact cause and align nicely with the company's bottom line and PR efforts.

Because it doesnt make money and doesn't cost much to ignore?

It's really not much different from SPAM filtering and not as easy as you suggest.