My issue with Facebook
I used to have at least one active a Facebook account, because of its conveniences, despite many ethical problems associated with using (or being used by) Facebook. But my biggest concern with Facebook is that it is actually not a "social network," because it actually does not facilitate sharing.
First is the problem of the authenticity of thoughts. After all, while we are having a "social network" that encourages people to share, but people don't really share what they really think anymore. People share news, but the news is not quite what people think. It turns out I can't share what I think at work because I have coworkers on Facebook. Likewise, I can't share what I think at home because I have my family on Facebook. In overall, I can't share anything because everything I say is tied with my real identity on this network. So in order to please everyone, I'd have to be the lowest denominator of every thought. This makes Facebook a bad platform to actually share and discuss matters.
Second, no one really cares about what you share. I consider a large part of the identity of a person is their interests and hobbies. Facebook does not have an effective way of promoting such interactions, either. It blends everyone a person knows to one category, friends. In real life, a person's friends or acquaintances might have a wide range of interests. Therefore, even an excellent content is unlikely to be well liked universally among a person's friends. I have friends who create contents who could find their audience on Youtube but couldn't manage to accumulate even an equal amount of "likes" on Facebook. Because people joining a forum or subreddit actively choose their interest, while on facebook you're at the mercy of the algorithm to tell you what you most likely to like.
The platform is extremely robust for messaging, so I will have to miss that aspect. There is a price to pay for everything, though.