Over the years I've ridden many types of mountain bikes configured in all manner of ways. For years I was a hardtail holdout, riding narrow 1.95" tires. Back then I thought 2.2" wide tires were really fat, rolled too slow and were too heavy. Perhaps one of the biggest abominations I rode was my Titus Racer-X full suspension bike with 1" wide road tires up Pikes Peak. Yeah, that drew some weird looks. I could only bring one bike to CO, and I wanted to ride up Pikes Peak again on a closed course from 6000 to 14,000ft. After the race, regular knobbies went back on the bike. Funny back then, mountain bikes were called fat tire bikes, presumably to differentiate them from road bikes.
An abomination! Not just the bike, the fanny pack too!
As the MTB community weened itself from fretting over grams, bikes got beefier, travel longer and tires wider. We were still just riding our bikes. There was no need to say "I'm riding my 2.2" bike today," or "I'm riding my 2.4" bike today." All that mattered is you rode.
But then came along winter fat bikes, which are just called "fat bikes" today. The old fat bikes, regular mountain bikes, are ironically call skinny bikes now. The fat bike Kool Aid was quite intoxicating. Everybody had to have one, even roadies that had never mountain biked in their life.
Suddenly, things changed. You could ride only fat bikes on groomed snow. Fat bike races appeared on the calendar, where, you guessed it, only fat bikes were allowed. Group rides were advertised as fat. Ride reports, Strava posts, Facebook statuses all had to say if a ride was fat or not. Now that the snow is gone, the distinction persists. Why is this?
I can think of several reasons. Perhaps the infatuation with riding fat bikes got so deeply embedded in our psyche that we have to shout out "fattie ride!" every time we ride our fat bikes. The bikes do certainly offer a different ride quality, and I can see how some folks like it.
Maybe it's something else. Fat bikes run with low PSI in the tires certainly don't roll as fast as our nimble, capable full-suspension 29ers. Could touting a ride as "fat" be some sort of "I'm not slow, I was just on my fat bike!" apology?
Sometimes only a fat bike will work in the conditions. Perhaps calling a ride a fat ride is explanatory for how you were able to accomplish the ride given the conditions. Certainly after a big snow dump, anything but a fat bike would be a miserable experience.
This distinction of fat vs. non-fat riding becomes apparent only via the social media. What if social media didn't exist? What if there was no way to exclaim to the world "we're on our fat bikes!"? If only the people you're riding with know what bike you are on, there'd be no reason to say "I'm riding fat today!" It would be obvious to them and they wouldn't care. You would just be out riding your bikes, like in the old days.