Jane Wong, a mobile app reverse
said that her Google search revealed around 470,000 results for WhatsApp invite links, allowing anyone to join the groups and access to members' phone numbers.
In all fairness, the privacy of these links is the responsibility of the admins generating them. By sharing them on the surface web - the internet that is indexed by conventional search engines - is a sure way to have them indexed by public search services.
Google's public search liaison Danny Sullivan
explained that this is normal behavior, the same as when "a site allows URLs to be publicly listed."
Using special search parameters, several users discovered that Telegram channels were in the same situation. It is unclear whether the admins made the invite links discoverable knowingly or in error. Regardless, some very unsavory results are not difficult to find.