I tend to consider finding a visible profile on facebook just a bonus, and don`t consider it to be a final word either way - we all know how many people have an alternate persona online...
When an applicant (a) uses facebook and (b) has a visible profile on it, that`s a score. otherwise, I make my decision based on the other (more reliable) methods of screening tenants; credit checks, current landlord, past landlord, current boss, past boss (if it`s been recent enough), etc...
Personally, I don`t want any of my tenants to consider me their friend, and would never invite them onto my friend list on facebook.
That`s just my thoughts on the subject.
David.