I think Facebook is the biggest cell phone business provider in the world. No company have 1B subscriber.
Although Facebook has app, I don't think it's convenient to use it as a phone service.
Cell phone service provider such as AT&T, sprint, etc have to change it's business model to data service provider, LTE or more, however, they are also going to meet competition risk with broad area Wifi service provider soon.
And, cell phone service will be serviced with free of charge, however normal cell phone service provider cannot provide international free phone call service because of counter company issue.
But Facebook can do it.
Facebook can provide free international phone call service and it can be free of charge. And there are several suggestions for doing this.
1. Very simple app only for phone call and chatting
2. Should use one's own number not name
3. recommended number composition : national code + current phone number. (if 202-202-2002 --> 001-202-202-2002)
4. using Skype tech, Facebook phone call reach person who don't have Facebook account (will be charged)
5. phone call service is free of charge however, user should listen Facebook provided ring-back-tone which probably ads.
6. Also have monthly fee version which doesn't have ads.
7. ads can be vary depend on one's facebook using style.(of course, one's commercial pattern should be defined by using Big Data)
Facebook can conquer cell phone service business. They have capability, manpower and money.
Like cell phone OS occupied by Androids and iOS, take a chance in the non-competitive market.
Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours. I wrote this article myself, and it expresses my own opinions. I am not receiving compensation for it. I have no business relationship with any company whose stock is mentioned in this article.