I use DeviantArt, mainly because there are so many users. However, you can view other people's art as well and there are many professionals on there-- you can even request critiques if you are a premium member, or ask for people to give you tips in their comments.
Also, you can buy and sell your artwork! There are many different types of prints- you can even sell a calendar, if I remember correctly.
In addition, they sell T shirts, stress balls, pins, have a forum, contests, your own personal blog and so much more!