Ask HN: Stories from migrating away from relational databases?

I am interested in stories from people who started out with a relational db (postgres, mysql, ..), outgrew it in the sense that it could no longer handle the amount of data and/or requests and then migrated to a NoSQL db. I am not talking about projects that used mongodb from the beginning "because it needs to scale", but stories from migrations from one to the other. What triggered the migration? What type of data did you have? How did the migration turn out? Did you get the expected benefits?

The reason I'm interested in this is that I hear it again and again: We need to use a NoSQL db, because postgres is not fast enough. Is it though? Did you try and it was actually not fast enough or are you talking about some hypothetical future performance benefits? I am sure there are cases where this is true, but I am just as sure that there are cases where a relational db is just fine.