Sorry to say, but any time I need a private repo I just head to BitBucket. GitHub provides tremendous value, I hope they find a good way to capture some of it.
On a more important note, you're probably not GitHub's target market. It's more like agencies and in-house dev teams.
BB is pretty great though, so aside from the social factor, the functionality is pretty much the same.
That's technically true, but ignores the fact that the target market is only paying GitHub because it's popular among developers. If GitHub upsets enough developers, they'll switch to something else for their personal projects, and start recommending other things at work. The type of companies that use GitHub are also the type of companies that listen to their developers about things like what source control to use.
(And non-profits: https://github.com/nonprofit)
By which you further validate the market for them and for investors.
I guess if you depend on non-git features, like issues, there's a chance that there's some lock-in. However, most of the ones I use have a migration strategy [0].
[0] http://webapps.stackexchange.com/questions/49729/how-can-i-i...