Honestly it all depends whether or not you want to sell your art, or simply post it up for viewing,

A popular site (the largest site) for art sharing is undoubtedly deviantart followed by Behance
http://www.deviantart.com/
https://www.behance.net/

Or depthcore is a less popular alternative (may be discontinued)
http://depthcore.com

You may also sell your work on both Behance and deviantART.
http://www.deviantart.com/sell/
(Behance you must have your own store open externally to the site and you just link the two, as far as I am aware).