If I choose social sign in, will HTC have access to any of my personal information?

Yes, HTC will function in a similar manner to many of the applications that use social network account.  For example, with Facebook Connect authentication.  Before you sign in with your Facebook account, you will see a legal statement on the same page.  Tap on the Facebook information link and you'll see the information HTC has access to.  If you are not comfortable with this, then please do not sign in with Facebook on the HTC Account screen.