Facebook launches a new email client today. If Facebook play their cards right they won't just be the primary social news feed. Facebook can be the de facto contacts list as well. This means more often contacting people via their Facebook profile, not by clicking on a phone number. This is significant because Facebook's now big enough and entrenched enough that it's difficult to compete with. Network effects and switching cost mean it's hard to leave, no matter what privacy scares or abuse come along. And Facebook won't let you export your social graph. Hence last week's spat between Google and Facebook about Gmail contacts.
(Incidentally, it is still possible to extract your email address book from Facebook into Gmail. Follow instructions here, but using GMX mail, not Yahoo.)