Last round's valuation seems to be $1.5B, so it looks like the acquisition price is 2x that so everyone hopefully walks off happy. I hope Accel, GV, etc. didn't have a crazy liquidation preference.
Liquidation preference usually just bases itself on the amount of money invested -- If Crunchbase is right, they've "only" raised like $280M, so even with 3x liquidation preference, they're well clear of the $850M number that would trigger any of the annoying rights that strip stock from common sharedholders. Everyone will have done well on this deal.