Social games companies – or, indeed, any Facebook application developer – using the social network’s Facebook Credits virtual currency have to give up 30% of revenues to Facebook. That’s a business model that works for games like FarmVille, but presents more problems for, say, selling music. However, it seems Facebook may be flexible going forward, as it tries to get different kinds of companies to use Facebook Credits. “We receive a fee of up to 30% when users make such purchases from our Platform developers using our Payments infrastructure. In the future, if we extend Payments outside of games, the percentage fee we receive from developers may vary,” explains a quietly-amended section of its S-1 IPO filing. How it might vary remains unexplained, but if the terms are good, we could see more bands starting to sell songs for Facebook Credits.