For some people, they’re more likely to do the right thing when they will be noticed and more likely to do the wrong thing if they’re not. Case in point: I’ve never seen someone in our neighborhood not pick up after their dog, but I’ve seen plenty of dog poop.
So #trashtag gives people a way of being noticed even if no one is around. Find a place with trash and clean it up. Take a before and after picture and post it.