Ask HN: What kind of database to use for a Facebook like social network?

I am a self-teaching beginner. I am not asking this question to build the next facebook but to understand when should a particular kind of DB be used. Does using a graph database for users and a NoSQL database for the contents, posts, etc, while using an SQL to establish the relations between a user and their contents make sense? Or is using an RDBMS always the way to go and only start looking for alternatives when you hit a wall?