|See also:||Purity Culture in Fandom, Receipt Blog, Your Fave Is Problematic|
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"Calling out" is to bring public attention to perceived oppressive behavior.
- Call-out culture on WIkipedia
History of Callout Culture
In the Livejournal era, callouts were rare, though not unheard of.
The use of callouts could actually be helpful in the case of actual predators who were known for sexually abusing minors, particularly young girls. One particular case involved a man named fort_kanji, who targeted young white women with sexually explicit e-mails and, upon receiving their contact info, sent them dangerous objects and demanded they masturbate with them. 
In cases such as this, callouts were seen as useful, protecting minors from a potential threat.
op steals bones
Callout posts became most popular on Tumblr, due to the ease and speed of signal boosting via reblogs. Callout posts tended to use social justice buzzwords and terms, and often included accusations of bullying, stalking, harassment, lying, and other perceived negative behaviours. Like the fort_kanji example, there were cases in which this could be a way to alert younger users of predators, or as a way to alert users of racist, misogynistic or other prejudiced behaviour by another user. However, those on the receiving end of of a callout post tended to lose their reputation, and a number of callout posts included false accusations and abused the signal boost system to ruin someone's reputation over a minor fandom slight.
The blog Your Fave Is Problematic is understood as being a major contributor to the rise of callout culture. The blog was known for its callout posts and for celebrities and other popular cultural figures. These examples (or "receipts") received a great deal of criticism for being frequently exaggerated, taken out of context, lacking nuance, or describing events the incriminated figure had long since apologized for.