I think a lot about how Facebook simultaneously fucks up and enhances our lives. Beyond the general knowledge that Facebook has completely changed the way we create and view friendships, I'd argue that it also transforms how we enter into and entertain relationships.
There are two levels to this:
The Poke, The Fuck
People who poke you want to fuck you. End of story. If you want to fuck them, you poke them back. It may never amount to anything except some drunken cyber sex, but there's always that chance you'll run into them at your mutual friend's pretentious loft party, recognize each other, somehow still think that your hotness translates from pixels to the Real World, and act on it a la a one night stand. But once you fuck, you can't poke. They're mutually exclusive. Poking is a lot like the frat guy who loses interest once he puts it inside you; it happens and then it ends once it leads to something real, leaving you to wonder if it ever really happened at all...
The Person You Meet Once, Hit it Off With, And then Subject Yourself to Nothing Beyond Online Contact for the Rest of Your Meager Lives
Maybe you met through another friend, or randomly at a club. You really liked each other, and once you friend each other you send flirty private messages and wall posts back and forth. You get excited when your profile shows Inbox(1). You graduate to talking on AIM, and soon to on the phone. It's getting serious. But it can never be because he was only visiting the city and he actually lives in Croatia and the time difference makes things hard and the distance makes things hard and without Facebook, you would have simply accepted the fact that meeting him was a one time thing. But Facebook makes one time things impossible. Facebook makes everything eternal. Now you two are trapped in this in-between, this never never land of messages and jealousy over relationship statuses and falling into e-love, but never being able to act on it. It is torturous, but you move on. You poke other people. Soon you rarely talk, let alone show your intimate poetry to each other. He becomes a ghost of a person, flickering sadly on your buddy list every time he signs on and off. You change your screen name and forget to tell him. He stays in Croatia and you move on to someone else you meet, a cyber crush, a Facebook boyfriend, a love, a lust that will never be, except for on teh internetz.