This is an awesome project, if not because it makes the concept of exporting your data from these sites very transparent, and approachable for the average person.
If you're comfortable with releasing your code, it would be awesome to have some sort of "offline" mode where users could opt in to downloading your scripts and running them locally. That way you'd be affording ease of use and allaying anyone's security concerns. Of course releasing code opens up a whole other can of worms (maintainability, license), but until then I wouldn't feel comfortable running my exports through your sites, as much as I support the idea. Others might feel differently though, in which case the site has a ton of utility.
The exporting scripts are pretty trivial just using the APIs each of these services expose. The hardest part is setting up the OAuth bits, which require a web server to host a callback endpoint. This makes it non-trivial for the average person to run the scripts and becomes a nightmare for support.
That said, I might open source it at some point just so that people can audit it and verify that I'm not doing anything malicious.
If you're comfortable with releasing your code, it would be awesome to have some sort of "offline" mode where users could opt in to downloading your scripts and running them locally. That way you'd be affording ease of use and allaying anyone's security concerns. Of course releasing code opens up a whole other can of worms (maintainability, license), but until then I wouldn't feel comfortable running my exports through your sites, as much as I support the idea. Others might feel differently though, in which case the site has a ton of utility.