From reading your previous questions and answers it seems that GitHub is pretty laid back with minimal management. In such a open work environment it seems like it would be hard to quantify performance and therefore measure who should get paid what.
Feel free not to answer, I am just nosy today.
For most fulltime positions we pay the same, base developer salary for everyone (contractors and some new hires are different, depending on the scenario). So off the bat we don't really fret about salary.
Money's not that big of a motivator anyway, as you might imagine. I mean, we get paid a reasonable salary for living in San Francisco, but everyone is much more motivated here by the work and the atmosphere.
In terms of performance, it's not too hard to keep tabs on people. Are they active in Campfire? How's their activity on the codebase? Do they represent the Company well? Things like that.