Hi,

you could use:

CMSContext.CurrentUser.UserSettings.UserFacebookID;

But there is still a possibility that user was authenticated using standard authentication form. This way of authentication is not allowed in case the field IsExternal is checked/true:

CMSContext.CurrentUser.IsExternal

So if the option IsExternal is true and UserFacebookID returns Id, the user is connected using his facebook account.

Alternatively, you could use

CMS.MembershipProvider.FacebookConnectHelper.GetFacebookSessionInfo();

If the above method retuns FacebookValidationEnum.ValidSignature, the user is connected with his fb account.

Best regards,
Ivana Tomanickova