The Guardian has an article on "gossip surveillance" where strangers report on social media private conversations they are not party to in the hope of exposing duplicity from the speakers in reference to a third person who is not present. This nesaseraly requires releasing publically enough details to identify the subject of conversation, and likely to identify the speakers as well.
Would these actions be covered by the GDPR, such that the uploader would be a data controller/processor and this behaviour be potentially infringing?
One defence could be the the processing was of "household nature", but I would interpret the case of the Dutch grandmother who shared publicly photos of their children and it was held that exemption did not apply because posting photographs on social media made them available to a wider audience. This would seem to apply even more here.