1. A method performed by a presence service comprising: registering a user of a first device to map a universally unique identification (UUID) of the first device to the user; receiving presence information, from the first device, in a data format that uses arbitrary descriptors; storing the received presence information in a presence system in a data format using arbitrary descriptors; receiving a subscription request from a second device to receive particular presence content of interest regarding the first device, the content of interest corresponding to only a portion of the presence information received from the first device, the subscription request including a subscription expression that identifies the particular presence information content of interest in a content-based format that is independent of the data formats with which the received presence information was received and stored and uses arbitrary descriptors wherein the content of interest is selected from current location, ability to communicate, availability to communicate, willingness to communicate, activity, holiday/working, sleeping/awake, mood, interests, intentions and wishes; validating the subscription request by determining if information contained in the subscription request represents an authorized subscription relationship; mediating, between the user of the first device and a user of the second device, an exchange of a cryptographic key that enables the presence information to be shared; communicating a portion of the presence information to the second device based on the portion of the presence information matching the subscription expression; and upon the user switching to a new first device, re-registering the user to map a UUID of the new first device to the user and update a profile of the user in a persistent data store.