It seems likely that what's happening here is some combination of:

1) Spammers using one domain and multiple sub-domains

2) A poorly calibrated ML model for spam.

It is worth noting that FB get probably the world's largest amount of spammers, fraudsters and general bad people due to the fact that they have an absurd amount of users.

Honestly, though, if this stays on HN it will get fixed, but it's deeply concerning that if this is happening a bunch of times (and it probably is) but those sites don't appear on HN, then this will not get fixed.

It's also possible that someone has weaponised the FB spam system against dreamwidth (which seems less likely to be me, to be fair).