Help me in making a multiplayer game from scratch with user profiles and facebook involved

Hello guys,
My plan is to make a real time multiplayer game with user profiles from facebook. Basically it will be just like 8 ball pool from miniclip. You can search for people, add them, challenge them, all their display pictures will be shown, leaderboards will be there. So my question is where do I start ?

For multiplayer I can use Unet or Photon, I get it, but how do I maintain the user database and other things like adding and searching and challenging. I don't want to install my own server, I can use third party stuff but don't know which one to use. Please help me guys, thanks a lot