I see this fairly often. Why not make an account solely for auth/signup on these sites. Most of the time people who say this talk about how they don't have the time, don't want to be on social media, concerns about privacy, etc. If you have an account just for logging in to FB Connect, it seems like the problems are alleviated.
I think the concern here is that, even if you make an account specifically for the purpose of authenticating to other websites, you're still telling Facebook, Google, Twitter... which websites you're signing up on, which they of course have an interest in knowing so that they can better profile you as a user.
Note: I have accounts on these sites and actively use them, just my guess as to other peoples' rationale behind not getting an account.
