You need to know who to listen and who to ignore. Some people on these sites are just stirrers. Then there’s the pedantics who obsess about the layout of your work to the point of rudeness (many sites don’t allow you to properly format your work because it would take up too much space – Youwriteon for instance) and who don’t actually comment on what you’ve written.
You will read things you don’t want to hear, but if they help your work you need to listen. Don’t get all precious. Leave your ego at the door.
Avoid tit for tat reviews. Getting personal is a waste of your time and energy. I try to avoid reviewing the work of someone who has reviewed mine, especially when they have been particularly hard hitting in their review, because if I say anything that might be construed as negative about theirs they will assume it’s a tit for tat even when it’s not.
Be helpful and constructive when you review people’s work. Even if you think it’s not up to scratch, try and balance your comments with the good and the bad.
Be polite and encouraging. Treat people the way you would want to be treated.
If you expect to be told how wonderful your writing is, steer clear of these sites.