I have no idea. I believe they will start realizing that click-based advertisement is an increasingly unsustainable and flawed revenue model and will look for a ways to start a significant portion of revenue from another source. Assuming that:

1) They don't have time to build it. 2) Since, they will need to buy it, it should be a business that already generates significant revenue, thus the biggest acquisition price tag 3) The longer they stick to click-based ad revenue the lower their chances to diversify and the higher the chance that someone else comes forward with a new revenue model that will irreversibly undermine Google's.