by John Furgurson for Marketing Profs I guess vanity's not technically a sin. It didn't make the list of the seven deadliest, anyway. But the "look at me" approach to messaging and copy is certainly one of the most common mistakes in marketing. How many times have you seen a social media post with a look-at-me selfie and a headline like "We just got our product on Amazon!" Or the equally meaningless "My book's an Amazon bestseller." (For a five-hour period in a tiny, esoteric category.) Who cares?! Many people would argue that's what social media is good for, but the "me, me,…