I know it's ironic given the larger point of this post that I came out of it with a tangential technical question, but... Can anyone elaborate on this?

Databases. When you have 500 million users, you can't afford unique constraints and foreign keys in the db layer anymore.

Is he just saying that fully featured RMDBs don't have the performance to handle that scale? It seems like in some applications those checks will be necessary, and often the db will be the most efficient place for them. Not everything makes sense as nosql. Maybe I'm missing the point though.