Personalization is one of the key components of search relevancy. It’s not the only component (Google uses over 200 signals for a reason), but it’s a pretty big component. If results are tailored to you specifically, there’s a good chance they will often provide relevance to you. One of the best ways to achieve personalization through search is to get you results based on your established connections with other people. Google knows this. This is why Google is putting more emphasis on social search.