All the YCombinator companies are going to deflate.
AirBnB - https://news.ycombinator.com/item?id=10913151
Stripe - http://venturebeat.com/2015/09/17/paypals-braintree-is-now-l.... Braintree was bought only 2 years ago for $800 million. 6x less than Stripe's valuation.
Dropbox - http://www.theverge.com/2015/9/22/9372563/dropbox-really-is-.... They are bragging about the number of users, but I bet many of these 'users' are fake accounts made to get free space from referrals and never used again. Plus, there is so much direct and indirect competition. Their privacy policy is also very bad.
Zenefits - http://fortune.com/2015/11/11/snapchat-isnt-the-only-startup...