Normally this happens when a duplicate account exists, one with a spotify username / password and one with your facebook details.
Did you get an email receipt for your subscription purchase? In it, it shows your Spotify username:
Logout of the app, and be sure to use it to login to your account. It's the one with the Premium features. If you see a random number, that's your actual Spotify username, which was assigned automatically to you when you created your account through Facebook. If you see it in your receipt, you need to login with your Facebook details.