That said, if you're an insider still, you probably feel it's going to get better, so you hold. You're used to shares being illiquid and it's easy to continue seeing them as such. Plus, you get some bonus cash with follow-on RSU grants that are certainly being doled out.
I personally would probably recommend to cut losses and run - it looks more like Groupon, Gopro or Twitter right now than anything else.
Who is to blame? CEO and underwriters for overpricing their IPO? To me it looks like Snap was simply strangled by Instagram in the worst moment. The last private round was priced when it still looked like Snap would continue growing explosively, IPO was hence massively overpriced and employee options as well.
It probably is not helping company moral right now that the CEO gave himself an $800 million bonus for the IPO.
Source (paywall but free figure at the top):
https://www.theinformation.com/soon-free-to-sell-few-snap-em...
(In other words pass losses on to employees and guarantee [as much as possible] returns to investors despite risks.)
And yes, I am aware of the inherent discount for ISOs, so as an employee you still stand to make about 2/3rds the value of your options if the company is worth about as much as it was when you joined. This makes me a little suspect of the stated $15 strike price for ISOs, that doesn't sound like the typical 2/3 discount unless if the stock was privately worth $30-45/share at grant time.
Disclaimer: myself.