subposting
Subposting is sort of like subtweeting but on any platform of social media. Most people subpost with the intention of secretly talking about or referencing (typically a person) that they are angry or annoyed at without directly naming or tagging said person(s). Sometimes subposting can be in a positive manner but usually not as it is an activity usually used by passive-aggressive people.
I was subposting about Monica on Instagram after she was gossiping about me behind my back.
Subpost
Similar to subtweet but done on Facebook to talk about a Facebook friend by vague-posting behind his or her back.
I think he subposted about me with his vague status "I KNEW he would be there."