Ok, I signed up for MapMyFitness a couple months ago for a Military challenge and I see my rides are also being shown on MyFitnessPal. Not sure why UnderArmor needs so many sites that more or less do the same thing and I am betting go to the same database(s).
Garmin does push activities to 3rd parties, I run my Garmin 920xt along with Zwift and they both show up on Strava.