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.
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. 14. The method of claim 11 , further comprising, for at least one mobile user in the set, inferring at least one activity performed by or of interest to the at least one mobile user based on the single refined location context category.