As an interesting side effect, they will have pretty exact stats on how many active users they have; might help them sunset old accounts or move them to the slowest servers.
(Because of the offline nature of most git actions and different habits on pushing/pulling, it's probably hard to otherwise estimate how much a user cares about their github.)
Some people try to pool local commits into larger, less frequent pushes and pulls so the number of push/pulls is perhaps less relevant than their cumulative size. But pushes/pulls will never correspond well with user involvement because people use github for all kinds of scenarios. For instance, I might be developing branches that I don't want to push or pull to github yet--maybe I don't even intend to ever make them public. However, I may still want people to clone from my github repo and report issues to me.
The amount of time between someone getting an email from github and re-activating their account is probably the best metric github will ever have on users' attachment to their accounts.
From the way their servers are laid out in the presentation I saw they wouldn't want to move them all to a slow server, but would want them evenly distributed to help decrease load on the servers.
(Because of the offline nature of most git actions and different habits on pushing/pulling, it's probably hard to otherwise estimate how much a user cares about their github.)