13. The method of claim 11 , further comprising categorizing the mobile users into multiple groups based on similarities between the mobile users.
11. A method performed by a computer server for inferring location context categories for a set of mobile users having at least two members, comprising: for each mobile user in the set, sending a respective query to one or more mobile web applications that return at least one location context category applicable to a submitting one of the mobile users responsive to a mobile user location input specified in the respective query; for each mobile user in the set, receiving the at least one location context category from the mobile web application responsive to the respective query; and applying multi-user collaborative machine learning with an objective function to the at least one location context category for each mobile user in the set to infer a single refined location context category for each mobile user in the set to form a matrix.