a distraction from feelings of depression seems to me better than focussing on it online with chat rooms and forums
I think you're misunderstanding how teens use online support groups. Obviously I monitor DD's group chats with these people, so I see what they chat about (bands, anime, gothic or emo fashion, hairstyles and how unfair your mother is for not allowing you to shave your head)
In between the normal teen angst and chatter they understand and support each other in worries their RL friends might not understand (school trips - being trapped on a bus with 30 odd other kids, having a panic attack in the middle of science, the urge to cut yourself or purge) but that isn't the focus of most of their chats.
These are children who all gravitated towards the same vloggers, people they felt they had something in common with. Even the vloggers themselves do not vlog solely about bullying, depression or EDs. DD followed people who dress like her, listen to the same music, watch the same anime etc and as such found a group of friends who also enjoy these things.
I think having access to someone your own age, who understands why you want to carve your arm up with your compass is far more valuable than watching a shitty musical but less valuable than RL, professional support.
It needs to be monitored by parents of course, there is always the danger that these children start encouraging each other to follow extreme diets or help each other hide evidence of their self harm, but if used correctly, they can be a lifeline to an isolated teenager who believes they are the only teenager in the world who has ever suffered MH issues.
Thankfully we haven't had to deal with any of the bad stuff yet. The group DD is involved in includes some older teens who've come out the other side who police anything that starts looking insidious.