As I understand it, > (I) shared between the sender and the recipient; 171 (II) only visible to the sender and the recipient; and 172 (III)

As I understand it, > (I) shared between the sender and the recipient; 171 (II) only visible to the sender and the recipient; and 172 (III) are not posted publicly; maybe sender and recipient can be considered a group chat with multiple recipients? Otherwise you wouldn't classify something like Kik or Discord as social media.