A search engine index measures and tracks one or more numeric indicators of activity at different times, including a most current time, in on-line forums and social networking sites. Such indicators may include, for example, a current volume per unit time of messages or data posted to the site, a moving average of such volume, a moving average of current participants over time, a count of current participants, other indication of site activity, or any combination of such indicators. The search engine is responsive to queries seeking forums ranked by activity for a defined time period, including a most current time. The search engine prioritizes search results to identify the specified sites or locations in order of activity (among other possible criteria) relative to a defined time.