I used to be on one, haven't been on it for years, because I was trying to get better.
It's like this site - it has many sections, including recovery, many different eating disorder sections, mental health and what not. It was also like this site in terms of support, but also come can be encouraging, others not - a very mixed bag.
When I first found it, it was VERY encouraging, but after a while, it got a lot of bad press (much like this site), and so it must've added recovery etc, and from there, it seemed to be more support based.
And unfortunately new users 'wanting' an eating disorder, because they thought it was 'cool' - I think deep down, that wasn't the reason, they were struggling badly, and they wanted to 'fit in'. Yes, I know how messed up that is, but when you're very young (teens etc) you want to fit into the world, so I understood it, but I didn't encourage anyone, I was supportive.
I was struggling myself so badly with my eating disorder (a lot better now but still there - always will be), but I came off the site for good (very gradually), because I found it very 'triggering'. I sometimes contemplate rejoining, because I have no one to talk to about it. I still might, I don't know. Don't hate on me, I'm a grown adult, I make my own decisions, and as I said, it's more supportive than not now.