Kato's comment is on point. Being super new to Firebase can be a bit daunting so here's some ideas:
You cannot use the Login & Authentication in the way you want. It is a function that allows you to create users that can access your Firebase and that's it.
If you want to do more with users, create a /users node that contains other stuff.
users
uid_0
name: "Frank"
friends
uid_1: true
uid_2: true
uid_1
name: "Kato"
uid_2
name: "Bill"
This tells us that Frank has two friends, Kato and Bill.
To accomplish your task, Firebase will need to notify your app when new users sign up (by adding them to the users node and observing the Users node for .childAdded) and then display the new users to your user so they can add them to their friends list.
Posts can be handled in a similar fashion
posts
post_id_0
msg: "Hey look kids, there's Big Ben, and there's Parliament"
posted_by: uid_0
viewers:
uid_1: true
This structure has a post reference (created by autoId), a msg and who posted it and who's allowed to view it, in this case user with a uid_1, Kato.
Saving Data and Retrieving Data are critical reads and simply going through the well-written Firebase Guide to the Stars is a must.
Hope this provides some direction, and in the future, craft your code, create a Firebase structure and when you really get stuck, post those (as text please) as they will help us, help you.
