1. A method comprising: monitoring, using a computer, forum activity in a selected group of online forums accessible over a computer network, wherein the forum activity is defined based on an aggregate of user-contributed content by all clients participating in the online forums; generating an index of forum activity measurements comprising activity measures for each of the online forums, using the computer, wherein the forum activity measurements indicate measures of the forum activity per unit of time for different past time periods; receiving a data request for the forum activity measurements pertaining to one or more online forums included in the group of online forums from the index; and serving the forum activity measurements from the index in response to the data request.