tablecloth math time: $2bn worth, assuming they're worth 10 times their revenue (let's be generous), that would require $200m in sales. they have 10m users, assuming 1% are paying, that means 100k users for $200m sales, meaning each user must spend 2000 dollars per year at Github. Sounds a little high considering the basic plan is $7/month, but with entreprise sales and growth rate, it sounds feasible.
Google naturally comes to mind. Facebook / Parse, also. Amazon Web Services. Actually any company in this space (that is worth more than $20bn).
Why do you say an IPO is unlikely? I don't see why the public markets would be wary: Github is IaaS done right, has clear ways to make money and grow on its current offering, and can very well come up with innovations adjacent to its core services.
Well many companies are profitable (actually, it's the only prerequisite to stay alive). And arguably, Github isn't profitable enough to fuel its own growth, or they wouldn't need to take $250m from investors in exchange of shares. The question is: is it worth 2 billion dollars? As a developer my guts say yes, but I have no idea what their numbers look like...
Also, sometimes they have to take a lot of money, as later-stage investors want to control a sizable chunk of the company (in this case, 12.5%).