Friends don't generally charge each other money to stay at their place though, right?
Or maybe that's a difference between close friends and acquaintances--and many people have a lot of acquaintances for Facebook friends, so I can see it working out.
Maybe not a 1st degree friend but a friend of a friend or something like this.
A feature that finds out the "shortest chain" friend that rents out an apartment in your destination city would be interesting. But you need a social graph for that.
Or maybe that's a difference between close friends and acquaintances--and many people have a lot of acquaintances for Facebook friends, so I can see it working out.