It's unlikely we'll be adding private messaging in the near future. If we did it add it, it would be specific to chatting with your friends through our app when playing games.
Private message systems are grounds for unsolicited harassment, spam, and have other privacy and safety considerations. We feel that there are plenty of messaging tools that developers already use that help them manage issues like that.
If a developer or member of the community wants to be able to be contacted privately then they will make their contact information available on their page. Otherwise, you can publicly communicate with them on project comments and message boards.
Thanks