Flickr is an image host with some social aspects and community groups attached.
If you're after a service which hosts your photos online, can store up to fullsized images and will give you organising features for those photos then Flickr is a great service. It should be said that whilst they do a good quality free service their overall package is designed to be paid for (its somehwere around £20 per year so a very tiny fee).
The social aspects that it has are user groups and these can be for anything and everything. Many people fast encounter the "award" groups and these are certainly very strong on flickr. That said there are also great community groups on there, many brands have a community run group (Canon and Nikon ones are especially good sources of info); there are also specialist groups for film - specific camera bodies, lenses - even subjects and topics from nature to landscape to critique. You can even find groups centred around specific places or events - you can also start your own .
The groups are very much what you make of them, if you don't want to take part you don't and can just use flickr as an image host.
500Pix I've less experience with; however it does tend to advertise itself a bit more toward the "serious" photographer as opposed to just being an image host. It also has a vastly smaller population which tends to mean that the cream rises to the top more readily.
I would say that neither 500pix nore flickr has the "superior" photographers over the other - both have extensive communities and flickr for certain has the whole list from snapshotters all the way to advanced professionals.