As a hacker, sure, I'd rather not have this potentially spam my wall with posts. But as a college student living with other college students, it's the perfect choice. This could prove to be really useful, I'm looking forward to trying it.
Yeah, the facebook authentication is kind of a double edged sword. We want it because it will make it easy for users to invite their roommates, and more importantly, it allows us to have the concept of an "identity" to tie actions to. Going forward this could let us create a sort of roommate-resume tied to your identity.
Instead make it really easy for one roommate to create everyone's accounts, then you reduce the conversion requirement to just 1 person (though you do slightly increase the friction on that one person you radically decrease it on the others).