Looking at the cap table (to the extent it's visible in the S-1) it appears that the big winners are the investors: Breyer, USV, Accel, Tiger Global, and Index add up to 92.3% ownership of the company. [Edit, 65%]
Or am I totally reading that wrong? (Note, I really might be, this isn't my area of expertise. I just scrolled down to "Beneficial Owner".)
Edit: It was pointed out below that I double-counted Accel, making the total closer to 65%. Thanks!