The problem is the 'embed' functionality that's built into 500px. In the social networking box, there are your standard 'like' on Facebook, 'tweet' on Twitter, Submit to Stumble Upon and all that jazz.
One of these buttons reads 'Embed', and gives you a HTML snippet that makes it easy to embed photos into your blog. In fact, the Embed code goes further, and actively encourages it: "Copy the code to your LiveJournal or Wordpress blog".
I don't want to be difficult, but I haven't given permission to 500px to dissaminate my photos like that. Not without my permission, not without a licence in place, and (probably) not without paying me.
If I find a series of my photos on someone else's website, where they are being exploited commercially, I'd send them a takedown notice and an invoice.
The problem, then, is that 500px seem to be encouraging its users to commit copyright infringements of my copyrighted materials. They claim, apparently, that it is "good exposure" for the photographers. Personally, I strongly disagree - I'll decide what is good exposure for my own photos, thank you very much.
Right-clicking on a photo on Flickr has a completely different outcome: You get a pop-up reminding you that the content is copyrighted.
Against their own T&C
Okay, I'm showing off my geekdom properly, in admitting that I am actually reading the Terms and Conditions on the sites I visit, but allow me that.
On the 500px site, in the Terms of Service, it states "By submitting photographic or graphic works to 500px (...) you agree that this content fully or partially may be used on 500px web-site for promotional reasons (such as photos at home page)".
There is no mention whatsoever about re-distributing my images to a wider audience, whether via blogs, LiveJournal, Twitter, Facebook, etc. Enabling (nay - encouraging) users to embed my copyrighted photos on their own blogs, then, is against 500px's own terms and conditions.
Thanks, but no thanks
I understand what 500px is trying to do: Opening the internet is admirable, and wanting to share content all over the place is a great idea. But it's only a great idea when you own the content, and when you've decided that this fits in with your business objectives, and your approach to copyright.
500px deciding to share my photos with the world, encouraging people to commit copyright infringements via a feature I cannot turn off, is not my idea of a well thought-through website.
Let me turn off Embed feature, at least...
...Unless you are happy to receive a ream of invoices from me, of course. In which case, carry on, and could you send me the address to your invoice payable office, please?
Hat tip to @phillprice for the tip re: this article