I'd have to unearth the screenshot I took some years back, but that strays from the point, doesn't it? They weren't asking for home number, work number, and mo

I'd have to unearth the screenshot I took some years back, but that strays from the point, doesn't it? They weren't asking for home number, work number, and mobile number. Or as you mentioned: daytime number, evening number. It was more along the lines of: Name, Address, Mobile Number, Company Name, Job Title, etc. They specifically wanted a mobile number and no other type of number. Were they wanting a mobile number so they could more easily research an applicant using their internal tools? Only they would know. But it drew my attention to the possibilities and the importance of obtaining a mobile number. Back to OP's question, part of the reason a mobile number is required to create a Facebook account may be to help tie different types of records together. Of course, as mentioned, there are other reasons, such as security. Users will understand and provide a mobile number for that reason alone.