Post Private Message to Selected User
here is a rough sketch of how you could go about this:

iOS Side: Add a node using DIOSNode's add

Drupal Side:


Create you own custom content type, e.g. CustomMessage, with a UserID field for collecting the target user of each message. This could be a simple text field, or an Entity Reference field, single or multi-valued.
Create a Services View that will use a user's UID as a contextual filter, in order to display all messages targeted at a specific user.