With Flickr you can get out, via the API, every single piece of information you put into the system. Every photo, in every size, plus the completely untouched original. (which we store for you indefinitely, whether or not you pay us) Every tag, every comment, every note, every people tag, every fave. Also your stats, view counts, and referers. […] Flickr actually goes a bit farther, not only can you get your data out, but it gets enriched as it passes through the system. […] This isn’t the exhaustive list, just a few of the things Flickr does to respect, and collaborate with the people who share their time and data with us.
Anyone who writes a web service should be aiming for this. Anybody who uses someone else’s service that doesn’t provide this level of access should be asking why.