Facebook runs MySQL. I don't know if 400K people means 400K users for this system: I don't think so, but still Facebook is orders of magnitude greater and uses MySQL.
Here is the fact: scaling is hard. If a NoSQL backend was enough to scale easily, noone nowadays would really start with a relational database, don't you think? I know this is not really an answer to your question, but I think there is simple no answer to this.
Use whatever you are comfortable with, use what excites you most, use what you think you'll be using for the next years, or use what you think it's easy to buy support for. Don't care scalability in the account for this choice, because until you face a problem you can't know how to solve. You can't even know if it exists altogether.
BTW, there are lots of considerations about fault tolerance, caching and other things which will have a way greater impact on your performance than SQL vs NoSQL. BTW, not all relational database engines are the same (neither all of NoSQL datastores are...)