Is a TB really considered Big currently?
I maintain a database that's over 1TB and Oracle handles it very well. The trick there is understanding that, under absolutely no circumstance should you ever need to do a full table scan, because the table isn't designed for that.
So, I'd argue that a monolithic database is okay even up to 10TB even with slower disks as long as you never need to touch more than 10% of it. If you need to touch 100% of the data 100% of the time, I'd say anything over 100GB is too big for one machine.
The reality is that it just depends. There's times where you're going to want a hadoop cluster even for only 16GB of data, and there's going to be times where a database is going to be fine with 10TB of data.