Hot on the heels of the explosion of social networking is the emergence of a new market dubbed “social money” – the informal currency between friends. PayPal, MasterCard, Cingular, and a host of startups are offering a free service that keeps track of money owed between friends, critically they play an impartial role, removing the embarrassing nag-factor and ensure everyone knows where they stand and no-one forgets to pay. Gas bills, lunch, holidays, group gifts, literally anything can be tracked and who has and hasn't paid is crystal clear. Logging expenses is simple - via a text message to the site, followed up by a more detailed online entry the next day. So how do the sites make money? Their revenue model includes collaborating with financial institutions and telecommunication companies to offer settlement services to this entirely new market. Given people carry mobiles more than wallets this could be the next big thing::