I didn't find anything specific to open source, but in their FAQ[1] they say:
10. Is your API free?
Yes, our API is free, requires attribution and is subject to Rate Limiting. If necessary, these limits are also expandable for free through our Whitelisting program.
10. Is your API free?
Yes, our API is free, requires attribution and is subject to Rate Limiting. If necessary, these limits are also expandable for free through our Whitelisting program.
[1] http://developers.face.com/docs/faq/