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For me, the first thought of social media headshots was in an email exchange a while back with fellow blogger Ben Popken. He was sporting a cooler-than-thou avatar pic, and I asked him about it.
He told me that he "had it done," by photographer Nikola Tamindzic [NSFW]. In an instant, this made total sense to me. We register visual impressions in a fraction of a second, and Ben was smart enough to make the most of that for his online presence. Think of the number of images that hit you on a given day, and how efficient you are at making subconscious assessments based on image content and style.