Feature selection for tracking user activity within an application
Well, this may not answer the question thoroughly, but since you're dealing with information retrieval, it may be of some use. This page mantains a set of features and associated correlations with page-ranking methods of search engines. As a disclaimer from the webpage itself:


  Note that these factors are not "proof" of what search engines use to rank websites, but simply show the characteristics of web pages that tend to rank higher.


The list pointed may give you some insights on which features would be nice to select. For example, considering the second most correlated feature, # of google +1's, it may be possible to add some probability of a user making use of such service if he/she accesses many pages with high # of google +1 (infer "user context"). Thus, you could try to "guess" some other relations that may shed light on interesting features for your tracking app.