A fusion between the words "Market" (verb: to engage in the commercial promotion, sale, or distribution of) and "Avatar" (a computer user's representation of himself/herself or alter ego).
Marketars are used on social networking sites to give a voice and face to a commercial / promotional agenda.
Though a Marketar speaks with a singular "voice" - it's actions (content creation - i.e. photos, comments, etc) may be created by more than one person.
Our nightclub set up a Marketar on Facebook to help promote our events by posting party pics and letting folks know about upcoming specials.