"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."
Its interesting that many of my post-college friends are finding content on Facebook increasingly less relatable, and therefore using it less. I think part of Facebook's appeal to those still in school is that it acts to reinforce the social bonds that are formed through physical encounters. Once those physical encounters die out, Facebook's use is also diminished.
"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.