It should go without saying that Treasuries are risk-free since the government can print dollars to pay back its debts. But in this economy, investors are willing to get negative real returns (i.e., after inflation) on Treasuries since they're more concerned about the return of their money than returns on their money.
(For fairly approachable commentary on the current macro investment climate, I'd recommend reading Bill Gross' monthly columns at http://www.pimco.com/EN/Insights/Pages/InvestmentOutlookOver...)
It doesn't look to me like the people who love and use github have been using it because it looks like it will be stable, and stick around for a while. They seem to have been won over by popularity effects and performance/cool tech factors.
So, if github fails, it may not really be as big of an issue. Github could fail as an investment or company, and still leave a running system behind, up as a non-profit even; the risk to its customers doesn't seem that big. And the potential rewards, depending on how they invest the money, can be really big. There's lots of good work they can still do, that everyone will benefit from.