Please put thought into your posts. Writers here are looking for an honest critique and one-liners like, "Great job! Keep up the good work!" are doing nothing of the sort and will get you excluded from the group. On the other side of things, authors, please be appreciative of the feedback you do receive, and be willing to tolerate constructive criticism. If you can't handle that, you probably don't belong here.
When writing feedback, post as detailed of a response as you'd like, but be sure you have a balance between the pros and the cons of said story. You want to point out where you think the author could improve, because without knowing what to work on, an author can never progress. But at the same time, include highlights of the story. We all need some encouragement, of course. ;)
Also, pay attention to any questions an author might have asked about his/her work and try to answer them accordingly.
And remember, as mentioned in the story guidelines, this community is all about being a two-way street. If you have time to post your own story, surely you have time to comment on others'. Try to post as much constructive feedback as you can, and others will (or at least should... us moderators will make sure of that) do the same for you. Even if you feel like you don't have much to contribute, try your best. And you don't have to post feedback for EVERY story, just stay active in doing so. Posting feedback will also improve your own writing because it will help you become more aware of what you are doing with your own works. But if you just take, take, and take without giving, unfortunately, you will eventually be excluded. It's not fair if only a few select people are actively posting feedback; they have lives, too. Thank you for understanding. :)