Presumably Amazon's gain is Google's loss
Or is at least in the longer term. At the moment it is possible that the continued enthusiasm of advertisers to transfer budgets to online will disguise what has to be a zero-sum game at least from a market share perspective.
More competition for Google ought to be a good thing, but a duopoly won't change much in terms of the balance of market power, nor improve the poor transparency and crap returns of online ad spending.