What to use – "Login" or "Sign in"? What about both?

I was trying to make up my mind whether to use "Login" or "Sign In" in the project I am working on by going through various well known websites. In my project, user can log in using either Facebook or email id. I landed upon Duolingo's and saw this:

Now, what? Why not "Login with Facebook or Login using Facebook? Why have they used different terminologies to essentially depict one thing? Or am I missing something here?