In Airbnb, I see that the user can 'sign up' with Facebook or 'sign in' with Facebook. What is the difference between sign up and sign in? Do these options actually perform different actions?
When you sign up with facebook, you're essentially just creating a profile on the local site (Airbnb in this case) using data from your facebook profile. So something like name, avatar, and location.
When you sign in with facebook you're authenticating yourself using facebook.
Although the two are related, they are essentially different actions, and so in my opinion, it's the right move to separate them.
Note, that although we've used Facebook as an example, the same could apply to Twitter, GitHub, Google, etc.
The risk of removing the sign up is potential user hesitation/confusion. If users only see "Sign in" -- they might think, "But I never created an account! Where do I sign up?!"
That said, I think it would be really interesting to do think-aloud usability testing and see how many people actually have an issue with only seeing a "sign in". My gut feeling is that not many people would. (Plus, the system on the backend would just be able to determine if it's a first time user and treat them appropriately.)