I would get out of the investment at the best possible price. Imagine you think your shares are worth $5 million right now, but you could only get out at $3 million. If you were not invested in this company, would you take $3 million cash right now and buy into half the company at a deep discount? If you really think there might be an 8 or 9 figure exit in the future, sell 90% of your investment and keep 10% as an option. Staying invested with shares heading to zero makes no sense to me.