Facebook doesn't make any money off that.
It makes money selling peace of mind to advertisers that 1. each account represents a unique cell phone subscriber, and 2. users are rich enough to afford a personal cell phone. And I don't know whether Facebook owns a position in one of the major carriers or vice versa, but if so, it'd make money when more people sign up for a cell phone.
Likewise, Facebook requires that you have a computer with an internet connection.
Facebook requires users to have access to a computer with an Internet connection, but it requires each user to have exclusive access to a mobile phone. One of several members of a household who share a POTS line, PC, and Internet connection has access to a PC but no unique mobile phone number. This is especially true in areas where the only wired broadband ISP includes a POTS line at no extra charge to DSL customers, like where my mother lives. Likewise, someone who accesses the Internet at work on break or at a public library has access to a PC but not necessarily a unique mobile phone number.