Sometimes I find I liked posts that I never seen/read before. Feels like a stranger likes posts instead of me!
My Facebook password is very secure and I'm sure no one has it but me.
Do you have any ideas why this might happen and is it possible?
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You probably leaked an access token to a rogue app with
publish_stream permission or leaked your credentials to a site you thought was affiliated with Facebook.
Changing your password and turning off the app platform https://www.facebook.com/settings?tab=applications§ion=opt_out&view should stop any suspicious activity.