This is actually a pretty good idea. Remember the discussion of a payday loan startup a while back?

Mobile phone companies give people very expensive stuff (high-end phones) in exchange for regular payment. This is effectively a loan, given even to people no bank would deal with.

Mobile phone companies can do a lot of damage to your social life (by cutting off the phone); apparently, this is sufficient to keep people paying their monthly fee. Again, this includes people no bank would deal with.

The parallel to Facebook, say, would be obvious. But it's hard to make this work if the borrower can just change his/her password...