2.1 Will anyone know that I signed up to Witnesser, or authorized Witnesser to access my data on Facebook, Twitter or Gmail ?
No, if you chose the right settings on Facebook.
This information will not be communicated to anyone by Witnesser. If you chose the right settings on Facebook, this information will not be accessible to anyone, unless someone has access to your email, or to a Facebook, Twitter, or Gmail account for which you authorized Witnesser.
If you don't authorize Witnesser to access your data on Facebook, you don't need to chose any settings on Facebook.
If you do authorize Witnesser to access your data on Facebook, and if you wish to keep it confidential, you need to go to Facebook's app settings
and check that Witnesser's visibility is set to 'Only me' :