I agree with adding some form of currency, unless you need to pay for it. It would be easier (and more effective) to have ads to pay for server costs or to gain revenue. If microtransactions are added, five-year-olds will be reaching for their parents' credit cards. Also, it would be difficult to implement microtransactions due to how profiles work, and people would be really upset if they lost their account after having done a microtransaction.