I discovered 500px.com today. This place really looks like it is alive. It is buzzing!
After spending some time (and disappointment) at so many other photography web sites, it took all of about 5 minutes before signing up for an account here. It doesn't look like it offers the groups and challenges of some of the other sites but I am still finding my way around so maybe I have missed something.
Regardless, this looks like the place to be :)