1. A method for recommending mobile device activities, the method comprising: receiving an indication of a content item contained on a Web page that is currently presented by a mobile device; determining semantic information about the indicated content item, including accessing a semantic network that is a graph data structure that includes multiple entities that each have is-a and/or member-of relations to other entities or categories of the semantic network, wherein the is-a and/or member-of relations are represented as links between the entities or categories of the semantic network, to: identify one or more entities in the semantic network that are referenced by the indicated content item and relationships relating to the identified entities; identify one or more entities in the semantic network that are related to the identified one or more entities; and identify one or more categories in the semantic network that are associated with the identified one or more entities and/or the related one or more entities, wherein the identified one or more categories are part of a taxonomic hierarchy in which each of the identified one or more categories is part of a corresponding taxonomic path that includes multiple categories related to one another via is-a relations, wherein the one or more categories are identified by traversing the links representing relations within the semantic network by traversing at most N taxonomic paths within the semantic network, where N is determined by user setting or data mining; determining a plurality of mobile device activities, wherein each activity has one or more associated entities and/or categories in common with the determined semantic information; and transmitting information about the determined plurality of activities.