A lot of the unicorns appear to have problems: they have a high valuation (probably good), but not the underlying revenue to support it (bad). With a few exceptions such as Uber and probably AirBnB, people are still buying on the hope of future growth. Or as Ceglowski would say, investory storytime.
Further, never forget that employees are plebes, and your and my shares are subject to a 6 month lockup that, for example, founders aren't necessarily subject to. See Mark Pincus dumping 16.5m shares of stock for $11.64 ($190m) two months before, and for $3.18 more, than regular employees could [1]. Zuckerberg also sold 6% of fb at ipo, though since the price didn't cater investors don't seem to object. Founders often can also take cash off the table. Note that I don't necessarily think that's bad, but as in the cases of Secret and Digg, employees don't walk away from failed companies with millions.
edit: actually, Pincus appears to have sold at twice the price available to employees ($12 vs $6.09), not $11.64 vs $8.46.
On April 3, 2012, the secondary offering closed at a price of $12.00. Mark
Pincus, Zynga’s founder and a director, netted approximately $192 million in
proceeds; the other three selling Zynga directors sold several million
dollars each in the secondary offering. Zynga’s stock declined significantly
following the secondary offering to a closing price of $6.09 on the date
that the original lockup restrictions ended, and a closing price of $5.36
and $3.00 per share, respectively, on the two extended lockup periods
applicable to the selling Zynga directors. [2]
[1]
http://www.law360.com/articles/596929/zynga-founder-can-t-sl...[2] http://www.law360.com/articles/608007/a-closer-look-at-delaw...