Late last year when an unknown group of hackers stole secret access tokens for millions of Facebook accounts by taking advantage of a flaw in its website, the company disclosed the incident and informed its affected users.
Similarly, when Twitter was hit by multiple vulnerabilities (#1, #2, #3) in the last few months, the social media company disclosed those incidents and informed its affected users.
And Guess What? Google is going to shut down its social media network Google+ in April this year after admitting two security flaws in its platform that exposed private data of hundreds of thousands of users to third-party developers.
It turns out that Apple also possibly suffered a privacy breach late last year due to a bug in its platform that might have exposed some of your iCloud data to other users, but the company chose to keep the incident secret... maybe because it was not worth to disclose, or perhaps much more complicated.