I posted a story earlier this week about Tagged.com. The New York Times also has an article on Tagged.com, MyLife.com, desktopdating.net, and what appears to be an accelerating problem.
….. I spoke to Greg Tseng, founder and chief executive of Tagged, to ask him what happened. He said all social networking sites invite you to e-mail your contact list to join up or discover which of your friends are already members, but that a software glitch meant an unusually large number of accidental invitations went out recently.
He said the company received almost 2,000 complaints from people who didn’t intend to send invitations to all their contacts — a fraction of the three million people who registered in the month when the problem occurred.
Read more on the New York Times.
Depending on how you define “breach,” this may or may not qualify, but I would view it as a privacy breach, as it is letting others know who is in your email contact list.