The full-size image is 1000 pixels high by 1000 pixels wide. Each pixel represents a number from 0 to 999,999. The row number gives the first three digits (0-999), the column number gives the last three digits (0-999)… so the pixels are ordered like letters on a page of English text. The pixel is black if the number is taken as a Twitter name, white if the number is still available.
Let me know if you’ve got any ingenious hypotheses, or if you want the data. (Although, in a very real sense, the image above is the data…)
[Secret bonus: Here's the image in the far more hierarchically-revealing Z-order (which I was delighted to discover has a name and Wikipedia page, since otherwise I would have had a hell of a time trying to explain it).]