I'm not a professional at helping others with depression and many users on Woozworld aren't. But I think if people are posting depression quotes, you should feel worried for them. That may have been their chance to reach out for help. Likewise if you see a real-life friend showing a even a small ounce of depression, you should be there for them. Depression doesn't heal on its own and you will feel better about yourself, knowing you've helped (and possibly saved) a friend. You don't have to force them to be happy, just let them know how much you care. It would also be nice to post encouraging quotes about feeling better.
However, if you don't know what they're going through, or can't find the words to say to really convince them, you should offer them professional help. If you are depressed yourself, you should also seek it. This link is a beautiful thought-out list of hotlines for anyone who needs them, listed by country:
http://im-just-lucy.tumblr.com/suicidehotlines
-FastWifiForever
