You may already have your account linked to Facebook, in which case this will unlock when you upload your first photo. If you do not, try to upload a photo after the end of the song and you will be able to log into Facebook.
If you do not have a Facebook account, you can sign up for free at www.facebook.com. Note that it does not need to be your main Facebook account that you link to. If you are not comfortable posting pictures of yourself dancing on your own account, you can create a new one with a seperate email account.