Sign-up or Login Security Issue

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Level 1

So I've been using Offerup for a while already and I have my account that I signed up for on the app on my phone and I've even posted right here on this community before. So I saw a few emails about my post in my email and wanted to go check what people have said since I posted quite some time ago. Anyway, long story short I opened up my broswer on my desktop computer, went to a link I copy/pasted manually to arrive at the offerup commnity page with the subject I had posted onto but couldn't see my post and It probably was burried several pages back. I looked on the top corner and there was a sign-in button so I figured I'd be able to find my post once I sign in. Clicked on the link and it asked me to create a username. I thought it was strange but sure, why not. Typed in a user name. THAT WAS ALL I gave it. Suddenly It had my picture automatically added to my profile and from the looks of things it create a new user account. Where or how did OfferUp Community get my info to create a new account without asking me for anything? While I'm very concerned this happened, it's not offerup I'm worried about, I'm more worried that some other tab I have opened allowed an authentication passthough, like the "Sign Up using Google+" or "Signup using Facebook" buttons that offer convenient single point signup and sign-in. I need to know which one did it. I don't see what email address this account is configured to use, where do I see that?