> Where is this meme that RDBMSs do not scale coming from?

They don't scale to Google or Facebook operational sizes. Once you get to a billion customers or so the ol' RDBMS tends to struggle. Because everyone wants to be Google they imagine they have Google's problems. I've been in a meeting where the client was talking about their severe scaling issues for their "big data" which could only possibly be resolved by state of the art cloud solutions. I pressed them on the numbers - they had 400GB. You can buy an iPhone with 512GB.

Oracle's Exadata X8-2 is ~1PB per full rack.