As an end user, I just don't end up knowing quite how my privacy is impacted when signing in through a third party site. I signed up to a site only yesterday t

As an end user, I just don't end up knowing quite how my privacy is impacted when signing in through a third party site. I signed up to a site only yesterday that gave me the option of signing in (or was that signing up - I haven't a clue...) with a yahoo account or a Facebook account, or an email address and password. I thought what the heck I'll sign in with my yahoo credentials. I then went back to Yahoo - and it gave me some message about authorisation and data sharing. What data is shared? Who knows. I didn't want anything shared, other than perhaps my email address. But why not explicitly say that? So for me it's just a case of pure confusion, and the worry and fear that I've shared something that I didn't want to with another application - for example I wouldn't want to share my address book. Another service I know of demands a Twitter login, as a result I don't use it. Which is annoying as I do want to use it. I can't be bothered to set up alternative Twitter accounts just for this purpose. I guess that if I don't get it - and I have a technical background - then what hope does anyone else have? Or do punters just go about and blindly trust services with their data?