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I don't think the flagging is there to use cause you disagree with someone, its more there for abuse and spam. But sure if you wanna waste your voting power on that instead...

She just told me to flag if I disagree with the post dude.

That's why I thought the flag option was there, else it would have been called unvote or negative vote or downvote. In my personal use I use it to flag stuff that I feel doesn't belong here, like plagiarism, spam, identity theft, etc. While others may want to use it to disagree on stuff, they are free to do so. I just think its enough not to upvote something I don't agree with or am uninterested in, then go and give my votes to the ones I feel deserve it more.