Facebook etiquette question

Question for longtime Facebook users: What’s the proper etiquette when someone you do not know asks to befriend you? On LinkedIn, most people I know will link only with people they know, because that linkage might act as a recommendation, an endorsement, a reference. What’s the case on Facebook? Is it like Friendster, with an arms race of friends, the more the better? Or is it like LinkedIn, where the relationships and history matter? And if I agree to befriend someone I don’t know, is it then proper and necessary to say I don’t know them? Or is that rude?
(signed) New kid on the block