8. A method implemented on a computing machine, comprising: receiving a plurality of candidate pieces of content; receiving a plurality of aggregated user activity streams, each aggregated user activity stream containing a record of at least one social activity performed by a user on a web page; extracting a plurality of topics from the plurality of aggregated user activity streams; calculating a velocity of each extracted topic based on a difference between a metric of the topic over a first period of time and a metric of the topic over a second period of time, wherein the second period of time came before the first period of time; calculating a trending score for each of the plurality of candidate pieces of content based on a measure of the velocities of one or more extracted topics contained in the candidate piece of content; ranking the plurality of candidate pieces of content based on the trending score; and publishing one or more of the plurality of candidate pieces of content based on the ranking.
8. A method implemented on a computing machine, comprising: receiving a plurality of candidate pieces of content; receiving a plurality of aggregated user activity streams, each aggregated user activity stream containing a record of at least one social activity performed by a user on a web page; extracting a plurality of topics from the plurality of aggregated user activity streams; calculating a velocity of each extracted topic based on a difference between a metric of the topic over a first period of time and a metric of the topic over a second period of time, wherein the second period of time came before the first period of time; calculating a trending score for each of the plurality of candidate pieces of content based on a measure of the velocities of one or more extracted topics contained in the candidate piece of content; ranking the plurality of candidate pieces of content based on the trending score; and publishing one or more of the plurality of candidate pieces of content based on the ranking. 9. The method of claim 8 , wherein the trending score is also based on a popularity score, wherein the popularity score includes a measure of social activity associated with one or more pieces of content each of which reference the topic.