Facebook UserName/Password. Be careful

I recently logged in to my facebook account through www.facebook.com

Later i checked with these below usernames and my password. It allowed me to log in?? OMG, is this a feature or bug or they are doing it wantedly???

sheshabootham@mydomain.com/mypassword

sehasbootham@mydomain.compassword

seahsbootham@mydomain.com/mypassword

shesabootham@mydomain.com/mypassword

Is facebook trying some crap? I know your data can be poached terming as agreement but what non sense is this?

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