Farrah Abraham sent eagle-eyed fans scurrying to cook up engagement rumors after she flashed a huge diamond ring on social media ... but TMZ has learned she's NOT someone's fiancée. Farrah set off the frenzy when she included a couple photos of her with a ring on THAT finger in a recent Instagram photo dump ... including one snap where she's pointing to the bling. The ring also popped up in a mirror selfie ... and Farrah further added to the engagement speculation when she mentioned "winning in

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