[croes]

The only way to prove it, would be a massive data leak.

[jhanschoo]

Or they can assuage concerned people with a 3rd-party audit.

[wellCrafted]

The fact there have been no data leaks should tell you more then some anonymous sources

[croes]

There is no fact that there is no data leak because it's impossible to prove. If the hackers don't go public to make money from the data you just don't know.