Unless you change your visibility settings, most of the information from your Eight profile is kept hidden from other Eight users until you connect on the App.
Your information may however be shared with carefully chosen and reputable third parties.
Cards you have scanned
Data from cards you have scanned in is kept private to you, and cannot be seen by other Eight users. Likewise, this information will never be shown to other users.
The only time this information may be partially visible to others is when new users are creating their account. It’s possible for them to search Eight’s database for their card at this time. In this case though, they must input the name and/or email address themselves, and they cannot see all of the card’s data until they verify their identity as the card holder.