This is the most compelling argument that I have seen so far from Benedict Evans on why we are not in a tech bubble. Highly recommended reading.
The state and history of US tech funding...... the current situation bears little resemblance to 1999 or 2000. This time it's different. But then, it's always different - what's going on now?
http://ben-evans.com/benedictevans/2015/6/15/us-tech-funding