Cyber-security company High-Tech Bridge set out to test the confidentiality of 50 of the biggest internet companies by using their systems to send a unique web address in private messages.
Experts at its Geneva HQ then waited to see which companies clicked on the website.
During the ten-day operation, six of the 50 companies tested were found to have opened the link.
Among the six were Facebook, Twitter, Google and discussion forum Formspring.
High-Tech Bridge chief executive Ilia Kolochenko said: ‘We found they were clicking on links that should be known only to the sender and recipient.
'If the links are being opened, we cannot be sure that the contents of messages are not also being read.
'All the social network sites would like to know as much as possible about our hobbies and shopping habits because the information has a commercial value.
‘The fact that only a few companies were trapped does not mean others are not monitoring their customers. They may simply be using different techniques which are more difficult to detect.’Ultimately, a communication provider (whether the post office, telephone carrier, or email carrier) relies on costumer trust that communications are and will remain private. That such communications are not undermines consumer confidence and trust, and eventually, the bottom line. It is amazing to me that companies will slit their own wrists, so to speak, by ignoring or acting contrary to their core mission or purpose.