Without prejudice to other definitions I would say that a web service is software system that allows for inter-operable machine-to-machine / application-to-application interaction over a network. This generic definitions would also help consider REST architectures to be a web service as they provide similar functionality albeit being an architectural style unlike the SOAP which is a fully defined protocol.
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You can build a profile on 500px, and photographers can sell usage of their photos through the site. However, you can’t directly link to photos on other websites. If you’re looking for a platform to upload photos to share on social media or other sites 500px probably isn’t the best option for you. The primary benefit of 500px is the opportunity to display your work and network with other photographers.