"The first time you launch the Facebook application, even before logging in, your phone number will be sent over the Internet to Facebook servers. You do not need to provide your phone number, log in, initiate a specific action, or even need a Facebook account for this to happen."
"The first time you launch the Facebook application, even before logging in, your phone number will be sent over the Internet to Facebook servers. You do not need to provide your phone number, log in, initiate a specific action, or even need a Facebook account for this to happen."
so an accidental launch is all you need.