Convertible debt as to not dilute the share pool. As is typical, there are also financial incentives to VPs to make the IPO happen in time. It would be very interesting to know if the lenders of the 1B anticipated this and are planning to immediately offload their highly discounted stock before shredding the company to pieces for their 1B. Might be hard, considering the massive liability they're taking on with the new New York office.