If you don't smoke, that doesn't prevent other people from smoking. If you're not on FB, that does prevent other people from messaging/whatever you on FB.
My God, you're right. I was never able to contact anybody before Facebook. It's a wonder I know other people at all.
Seriously, though, the biggest argument in favor of it that I hear is event invitations. But think about it, if the most effort that somebody can put into inviting you to their event is clicking a button to "send to all", they don't really value your friendship. I don't have a Facebook account, and when people invite me to something, they send a personal message that signals they actually value my friendship and want me to attend. As someone on the Internet said, Facebook maintains friendships in a persistent vegetative state.
Religion is vaguely similar... but the entirety of my argument is based on the literal social network and the strong ties and attributes that it carries. The value of individual social ties in a larger network.
I don't think that pot is related to this in any way.