I don't think they are bragging. They are contributing to the conversation with directly relevant content/personal experience. I'm not sure why you decided to be confrontational and condescending about that.
I also deleted all of my social media several years ago (except LinkedIn in case I need to find a job) to enormous personal benefit. Most people don't care when I bring it up. When I lived in suburbia, however, it was a different story. For every person that was interested or congratulatory when I told them I encountered five that were defensive about it and chose to frame it as some type of personal failing akin to alcoholism - "well I'm sorry that _you_ can't use it responsibly, but I can!".
I've no idea why this is and have more important things to do with my time then try to explain or rationalize this phenomenon but it was definitely a thing.
Mainly because I don't have an impression that it contributing to the conversation in a meaningful sense. We have a lot of different social media platforms out there. If you decided not to use one or more - good for you, but what exactly the message here? Could you explain it please?
I've no idea why this is and have more important things to do with my time then try to explain or rationalize this phenomenon but it was definitely a thing.