The fact of the matter is, there are some components of systems for which Redis, Riak, etc may be better suited than SQL in the long term. Starting out, keeping everything in SQL provides less friction, but as time goes on it may be necessary to scale the component separately from typical relational data storage, and that's the point at which these switches are evaluated. These companies would be replacing SQL only in these components — not across the board.
The myth of a silver bullet datastore solution is just that: a myth. Different data stores have different strengths and weaknesses and it becomes necessary to mix and match at scale.
To quote Benjamin Black: "Scale is pain, princess. Anyone who tells you different is selling something."