Staying off Facebook is not enough. Facebook could easily write a web crawler to find off-site photos of people tagged on Facebook. They could even create shadow accounts to track tagged people who don't have Facebook accounts. Even people not tagged could have shadow accounts seeded from Wikipedia or news photos.

The second information which I see having potential is the EXIF of the pics, the off pixels and other identifying data of the camera (flare, blur, watermark), artificially introduced or not.

Not only it proves who we with whom at the same time. It also links profiles. You can create a whole new identity; but if you didn't throw away all your phones and cameras, then Facebook can link the profiles. Or if you sell something on Craigslist and upload on Snapchat with the same camera: It's tied to your identity.

Was it just to make a point? Anyone can Google "Richard Stallman" to retrieve more than enough training data to train an image classifier.

There are more than enough pictures/videos of Stallman to make a really clean faceprint.