1. A system for selecting among a plurality of advertising content from a first and second mobile content inventory for presentation of the advertising content on a mobile communication facility, the system comprising one or more computers having computer readable mediums having stored thereon instructions which, when executed by one or more processors of the one or more computers, causes the system to perform the steps of: receiving at a server a request for an advertisement associated with a user-action initiated at a cellular phone operated by a user, wherein the user-action is a search query, a request for a webpage, a request for an audio file, a request for a video file, an SMS, or an action inside a client application operating on the cellular phone, wherein the request for an advertisement is received over a cellular telephony infrastructure provided by a carrier, wherein the user is paying for use of the cellular phone with the carrier; searching the first and second mobile content inventory for a first and second advertisement, respectively, that corresponds to the request for an advertisement; determining that the first advertisement is more relevant for presentation to the cellular phone than the second advertisement, wherein the determination is based at least on a yield optimization, wherein the first and second advertisements each have a yield associated therewith that represents an expected revenue associated with the respective advertisement, wherein the expected revenue is expressed as one of: (a) a cost-per-thousand impression (CPM) of the respective advertisement on a plurality of cellular phones; (b) a cost-per-click (CPC) of the respective advertisement on the plurality of cellular phones along with a historical or expected clickthrough rate; and (c) a cost-per-action (CPA) along with a historical or expected action conversion rate; determining that the yield associated with the first advertisement is greater than the yield associated with the second advertisement; and transmitting the first advertisement instead of the second advertisement to the cellular phone for display thereon.