I just come to realize that I can login to Stackoverflow with my Facebook account and still get my same SO account as when I log with Google OpenId (which i always used before) but... I never added Facebook OpenId to my SO account (in my profile I only get openid : Google (email@example.com).
Also, when I log in with Facebook and I click to logout, my Google account is displayed like this
You’re a registered user.
You have the following credentials associated with your account, which you can use to log in at any time:
Nothing about Facebook there even if I logged in with it.
Can someone explain how's that working ?