As someone whose moved around the states a bit over the past few years I'll chime in. At workouts in the local gyms I've definitely noticed trends. I was in the South (Texas, Alabama, Mississippi) and maybe 50% of the guys were wearing briefs, but only tighty whities. Only a few boxers. I don't think I ever saw fashion or bikini briefs. The other 50% were primarily boxer briefs and tended to be a little younger guys.
On the west coast (Oregon and Cali), I almost never saw tighty whities. If I did, it was an older guy. Same with boxers. It's pretty much all boxer briefs with a some fashion briefs mixed in.
In the north east (Mass and NH) I saw more boxers on guys of all ages, maybe 30%. Plus a lot of boxer briefs. A few briefs here and there, mostly fashion colored ones.
Haven't been to the Midwest much, so I'm curious what the trends are there.