1. A method, comprising: a computer system receiving data from tracking code injected in a web page of a first web site loaded by a web browser of a user computer, wherein the tracking code is executable to individually track visibility of a plurality of distinct items of user generated content (UGC) that are part of the web page, wherein the UGC describes experiences with one or more particular goods or services, and wherein the received data relates to individual amounts of time that each of the plurality of distinct items of UGC is visible on a display of the user computer; based on the received data, the computer system assigning a plurality of numeric impact metric values that each correspond to a respective one of the plurality of distinct items of UGC, wherein ones of the plurality of numeric impact metric values are based at least in part on the amounts of time that the plurality of distinct items of UGC were visible on the user computer, and wherein each numeric impact metric value is further based in part on whether the corresponding one of the plurality of distinct items of UGC was associated with a purchase transaction, and wherein the assigning includes: determining that an amount of time that a particular one of the plurality of distinct items of UGC was visible exceeded a threshold amount; and in response to the determining, precluding the amount of time from an assignment of a numeric impact metric value of the particular distinct item of UGC; the computer system receiving a request for UGC from a second, different web site; and the computer system determining whether to provide one or more of the plurality of distinct items of UGC to the second web site for display, wherein the determining is based, at least in part, on the plurality of numeric impact metric values.