The Top Content report in Site Explorer reports 2 different social metrics:
The number shown is the total number of shares on Twitter. A share on Twitter looks something like this:
The number shown is the number of "pins" on Pinterest. A pin looks like this:
These social metrics are taken directly from the social sites' public APIs.
Note: It is important to point out that we show social metrics for the exact URL, and not for the domain as a whole.
These values also hold true for those you see in Content Explorer: