Which Database to use for social networking android application?

I have recently started to built some android applications now I want to add a back-end database to my android application.

Can anyone tell me that which database to use and how to go proceed with it? I have read lot of articles and links about SQLite, Parse, MangoDB and other relational databases.

My requirements are like number of user would be more than million and most of the data shared by the users amongst all of them would be in text only. And all the users would be related to each other.

I know there are many articles out on internet but I am not able to find one stop solution, I am a beginner in this please help me.

Thanks!