Here's a counterpoint from Sam Altman of YCombinator:
http://blog.samaltman.com/the-tech-bust-of-2015
He thinks most of the bubble is limited to late stage private valuations and that even those "valuations" are being misunderstood. The late-stage unicorns raise primarily through debt instruments with high liquidation preferences; the top line valuation in those is actually more of an option.
Just as an aside, the only outside investor, Sequoia, made out like bandits on this deal:
April 2011 investment: $8M
Rough estimate of their stake now (assuming conservative 15% ownership): $2.4B
300x return in under 3 years.