500px.com isn't too bad, not as much variety as flickr though.I'd never really heard of flickr before I read about it on some threads on this site, but then again..I'm just now getting into photography.
I think it's mostly due to Yahoo just letting it languish, and not doing much to keep it fresh. Plus I think a lot has moved to facebook, etc. Hopefully with the new CEO at yahoo, they can maybe breath a little life back into flickr.
I did read that the new Yahoo CEO does have it high on her plate that Flickr will be getting updated and improved as one of the first items going forward.
I've heard there are lots of changes already...a more 'google' like environment at Yahoo now..hoping to capture and retain more talent to help the site improve.