I don’t know a specific site, but I think the parent should only recommend it if they monitor their child well regarding online usage, and insist it is done under a username, not their real name on sites like that. I also think the child should be in therapy concurrently.
I think it is extremely common for children to be depressed because they are lonely, and the web can help reduce the feeling of loneliness. I also think children suffer from self esteem issues, low self worth, and sites like fluther can help with that. I don’t think the child needs to necessarily be in a group, whether online or not, about feeling depressed, any group where they feel connected will help. Could be an interest they have, or people interested in their opinions, or helping others.