1. SELL. Develop applications solely for the purpose of selling them to interested parties. Several applications have already been acquired in this fashion; the most recent example is that of Mozes’ purchase of TextMe.
2. DEVELOP. An indirect source of funds: develop applications under contract for third parties. A number of companies have been posting contract jobs over at the Facebook’s developer forums. There exists a large gap in the supply and demand of available Facebook application developers; as a result, finding potential clients to charge reasonable rates should not be a hassle.
4. MICROPAYMENTS. Sell services within Facebook through micro-payment transactions. PayPal payments made for accessing premium services could potentially yield reasonable income depending on the application’s purpose, size, and prospective users.
5. GET INVESTMENT. If you think you have something big on your hands and lack the funds to scale it, apply for investment through Bay Partners and others who have expressed interest in funding Facebook apps. This would be the likely course of action for a startup with plans to expand globally within and beyond Facebook. source mashable.com