Personally, I don't mind. I assume anything I give to a social networking site [e.g. GitHub] is public knowledge or might as well be.

I mainly value my privacy about non-public information [e.g. lovers, close friends, current location] and that isn't acquirable without violating the privacy of my phone/email/etc.

While you may consider user e-mail addresses to be public, some people take umbrage when their e-mail addresses are disclosed.

Personally I use a different e-mail address for almost every site, so searching by e-mail address wouldn't find my account, but I know many people use the same e-mail address for several things.

True, some people use a different address for each service (ie. yourservice@mydomain.com), and they tend to be the the kind of people one would want to hire as engineers! I wonder if Sourcing.io figured it out somehow, but at least compared to option 1 it seems better to look them up manually.

That is what the parent comment basically said.

Yes, I do understand people in the world don't all share my opinions given I was replying to the opinion you present. ;)

Would you find it creepy if an employee were doing this manually?

Yes.