How long should you wait to Facebook-friend someone you met?

I’ve heard competing theories on this, that if the person is in your extended social network (e.g., in your grad school program, etc.) it’s acceptable to friend the person immediately after meeting them (say, when you get home from the party whence you met them). But in other situations, say someone who is not in your social circle necessarily, but with whom you “hit it off with” at a bar and they gave you their card, etc. and the meeting was purely platonic, a la “you’re cool, we should definitely hang out”, is it too soon to add them after only one encounter? Sometimes I feel that adding people too soon is perhaps too overwhelming and creates a chiller effect on future interactions. What’s the rule?