This points to an interesting correlation (but NOT causation) between high Google rankings and the amount of Facebook shares. While he stresses that Facebook shares do not contribute to the rankings because Google is blocked from this data, he points out that the correlation might make Facebook shares a good external indicator of what Google finds to be "good quality".

In other words, if people share something a lot it must be good. If Google's algorithm agrees with this then getting a lot of shares is a good sign you might rank well too.