Mention-spam
Abusing @-mentions to flood someone's notifications — either as harassment (many accounts pinging a victim) or accidentally (hellthreads). A recurring moderation problem on all @-based platforms.
Attacks: coordinated campaigns where a target is tagged thousands of times → notifications become unusable, a form of cyberbullying. Defenses: notification filters (followed only, only accounts with a profile picture/age), muting conversations, rate limiting. Mastodon and Bluesky have granular notification control partly because of this. Distinct from regular spam (DM/post): mention-spam targets the notification system specifically. Bluesky introduced detailed notification settings (2024) as a countermeasure. The hellthread is the accidental variant of the same problem.