What's worse is developer-level services that require Twitter to sign up, or sometimes even Github. Not all developers (see: me) have a Twitter or Github account.
Twitter has no official policy barring the same person from having multiple accounts (though Github does, for free accounts; thanks to niggler for the correction) --- so, if you want to keep your use of the service from being tied to your other online activity, you can just create a throwaway account. Facebook does; creating "throwaways" potentially puts your use of both Facebook and the other service at risk.
More seriously, neither is known for, say, tracking logged-in user activity on other sites (the way Facebook is known to do with "like" buttons).
"One person or legal entity may not maintain more than one free account."
https://help.github.com/articles/github-terms-of-service Section A7
I'd say that GitHub auth for a service aimed at developers is a reasonable assumption to make.
Sure support it as an option, but don't make it mandatory that you have a account with social media service X.
As long as its optional. I'm hosting my own Git repositories on servers I own, hosted in centers with fat pipes. I know devs working on sensitive stuff which shall never make it to any public / cloud / whatever. There are also developers who are not at all into the "open source" thinggy.
It's good if they conveniently allow GitHub for those who are using GitHub as long as they also offer other ways to login (say OpenID [even if it has its flaws] or good old email/password or whatever).