1. A system that ranks search results, comprising a processor executing the following components: a ranking component that determines relevance of respective search results generated from a search associated with one or more of a Usenet, a discussion thread, a blog, an archived community discussion, or a chat room via multiple feature-based relevance functions, wherein features of the relevance functions are based at least on one or more global thread properties comprising at least a thread depth defined over a thread comprising one or more messages that include at least a message core and a message body, one or more posting-specific thread properties and attributes of a person posting the messages, and wherein the attributes comprise at least a number of posting per time duration, a number of newsgroups posted to and a number of postings that have no responses; a function generator component that generates the relevance functions such that ordered search results are ordered based on their respective relevancies; wherein the search is selectively scoped based at least on a structure of the thread comprising the one or more messages, wherein further at least one of the features is based on inferred labels on edges between an existing message in the thread and one or more of a parent or child of the message, and wherein the labels determine a nature of a respective message in the thread and are automatically inferred from content of the one or more messages within the thread; a thresholding component that defines a threshold for an acceptable relevance level for at least one of the multiple relevance functions to mitigate providing non-relevant search results to a user, wherein the threshold is dynamically adjustable to adapt to the user's response to returned results by raising the threshold when the user rejects at least part of the returned results with a relevance above the threshold; and wherein the threshold limits the returned results based on system level constraints.
14. The system of claim 1 , wherein the threshold limits the returned results based on at least one of available memory or processing capacity.