There doesn't need to be either.
Facebook's popularity is taking a nose dive. Lots of people are leaving.
No one will be using Facebook in the next 2 years.
Are you joking? Regardless that people are leaving Facebook, there is quadruple the amount of people joining every day.
iOS should have deep integration with Facebook; much more than the need for integration with Twitter.
Simple Answer: Yes.
Oh and trust me; Facebook will be massive for many, many years to come.