What they will get out of this is your biometric voice samples. Right now, if the go'vt wants to track (spy on) you and you don't have a sample already in their database, they have to go through various sources to see who you are. ie. Pix from Facebook and voice samples from either Google (voice) or Apple (Siri). This way, FB can offer up themselves as a one-stop-shop for any 3-letter homeland-security agency wanting to track you via video (pictures to run against video tracking) or audio (a verified voice sample to run against echelon/omnivore/etc).