Send direct message between two users


My problem is the following: I would like to send data from a user to another user ( just to one user not more). My question is that how can I send direct message between two users. I’m using ionic v3, and firebase. I was thinking about maybe I could send the message (data) to firebase auth Uid. I found the push and th fcm plugins but I’m not sure they are the best solution for me.

Thank you for your help.