It goes both ways IMO. On the one hand you have communicating with people in real life which goes a long way for us dealing with our own personal psychology and our own sanity, but on the other hand we have this platform that's basically a massive communication channel for people all around the world, where they wouldn't have been able to communicate before. I think the potential is there to both bring people together and push them apart for both online and real life, this is still a very new thing the world is becoming accustomed to.
I wouldn't go so far as to say social media is toxic though, certainly there are people on it that can be like that but that doesn't make the entire platform the problem. Granted, there are downsides even if everyone got along, like you mentioned there is a very real possibility for increased anxiety or depression if we use it too much in the wrong way, or in the wrong frame of mind (and I believe it's surprisingly hard not to do so). I don't think that makes it a bad thing necessarily... I think it's at least a growing pain of the technology though.
I love how you said that its here to bring people together. Nowadays whenever i would ask this question you literally hear memes, funny videos, news etc. And yea maybe its too far fetched to call it toxic but lets just say that the dose makes the poison, right?
How do you think we should act upon said people that 'mistreat' the platform though?
Anything can be poison in high enough doses, that doesn't make it necessarily bad. Too much water will kill you, but we drink it every day. Too much social media could make you depressed or at least increase your anxiety, but it's still good in connecting people.
For people who mistreat or abuse a platform, I think we should act by not supporting them. We don't give them upvotes, we don't comment on their works, we silence them in chat if they're too toxic, etc. Basically peer pressure, let them know that this behavior is not ok and we will not put up with it. That's why social sites try to build in ways of reporting and dealing with abuse, like an upvote system or a hide option. But it's not perfect, and even the best of systems are prone to both false positives (abusers don't get flagged) and false negatives (innocent people are flagged), and that's where a lot of the "growing pains" I mentioned in my comment come from. Even YouTube is still experimenting with ways of dealing with this kind of thing, and you can see it every time a creator complains about the state of the system ("Is youtube over?", "my videos are demonetized!").
I think there's also possibly a little cognitive bias going on - we may latch on to the negative far easier than the positive, even if there's more positive present. If you saw 10 good posts about, say, gaming and all the positive experiences from it and 1 bad post along the lines of "EA is a corporate money hole don't buy from them!" which one might you remember better? Maybe both, but the bad post is more likely to stick in your head.