I think you're misunderstanding what I'm saying. I'm not implying they need social signals to better target ads. But to give better search results. (as in google + your world)

The competition was already doing this, Google was threatened. Bing had a deal with Facebook. Users were already using Facebook to look for information instead of Google. They tried to outsource social to facebook and twitter. That proved an unideal strategy. Social was too important for their business to rely on the good will of another company. Social is the future of search. Google saw this coming a mile away and got into social before they could become irrelevant.

Social is the future of search, this fact is independent of advertising. Even if Google had a different business model for their search engine. They would still need social. They didn't get into social because of ads, they did because search needs it.