There isn't just one social graph, there are many. LinkedIn maintains your professional contacts, while Facebook has friends and family (and yes, often many professional). Twitter has some mix, as well, perhaps with a different filter. Aggregators take these disparate contacts and bring them into one place. A new service uses several APIs to merge them and give tools to better understand and utilize your network of contacts.
Sustaining an effective engagement campaign can take a surprising amount of work. You could limit yourself to following the leads from the data you collect, but following without interacting first will get you a very low follow-back ratio. More importantly, if you follow and then forget people, they may follow back, but they’ll soon forget you. A high number of followers who don’t reply to, mention or retweet your account are of no value. True engagement is long-term and repetitive.