Simin - You are asking really good questions and I have struggled with the same things! My philosophy is if I join a group or service, I need to put in more time reviewing and commenting on others artwork than I do uploading my own images. I feel it is important to be supportive and learn from the expertise of others. Therefore, since my time is limited, I have chosen to only participate in this forum and Flickr for serious photography and Instagram and Oggl for every day captures. I really like this community for the support and insight into mobile photography. Flickr provides a way to join groups that have other interests.
I hope that you are able to find your way through the maze of photography sites available.
Thank you for following me on Flickr.