1) They only get access to the information you authorize them.
2) If this isn't enough control, there are chrome & firefox extensions to give you more granular control over the authorizations you allow, though disallowing certain things can cause issues sometimes.
Then you don't sign up... Startups aren't obligated to give us everything we want and they certainly don't owe us anything. I personally don't like signing up for services using my email and I don't like signing up using Facebook... However, I don't expect startups to cater to everyone with a non-majority preference. Either I make some sort of sacrifice to use their free service, or I don't use their free service.
Ok but this is a Show HN. If we want to give feedback and test the service we HAVE to use our facebook account and give our info when we don't want it and that's problematic.
Once again though, I don't think it's problematic. I often feel that a lot of times, people don't want to pay the entry fee or wait in line to get in (figurative analogy here) and then feel left out.
We're in the tech space... If I designed a new tool for XBOX Live stats and posted to Show HN, it would follow that you would have to 1) be an XBOX Live member and 2) give me your information... I see using Facebook sign in as an extension of Facebook and not the other way around.
Just a matter of opinion from all around, though, obviously.
2) If this isn't enough control, there are chrome & firefox extensions to give you more granular control over the authorizations you allow, though disallowing certain things can cause issues sometimes.