I left facebook because it became a place where people I knew spent their time curating a false virtual identity of themselves and seemed to become a place to establish social status rather than any meaningful conversation. I've avoided instagram for the same reason. Twitter seemed a good platform for open discussion, but now it looks like a pit of misery.
Mastodon is pleasant and friendly, but I find it hard to talk about the topics I am interested in. Like twitter it is more person-focused. What would be nice is if there is a way to cut across it easily in a more topic-focused way. Hashtags are present, but they don't have the feel of a forum. For positive or negative, there isn't any 'algorithm' on mastodon other than reverse chronology, and I think that does miss out on highlighting interesting topics and replies. Also there is no moderation or guidelines for hashtags, a feature which helped guide people to the right discussions in reddit.