microblogging is something used on a much more personal level while i think forum style social media is mostly for looking for answers and posting things you do, but microblogging social media sites are for that too. also, this is not meant to be offensive, but i think that since mastodon has been in the works for like a long time, it looks more polished than modern websites, so that can be an advantage it has
Forums are for narrower topics or subject, while the world is your oyster when it comes to microblogging. I’d feel limited on a forum site, whereas here I can follow everything from gaming to dog posts.