They both have millions of users across the world. They both have made and broken friendships and relationships, and they both have raised millions if not billions of dollars for their respective companies. And chances are that they're both so popular even your grandma knows about them. Gamasutra has written an interesting post comparing both World of Warcraft and Facebook of all things, and they say that the two are more alike than you might think: both enable you to create an identity, and use that identity to interact with others, and both give you a wide variety of options to do so (in WoW, you can slay dragons together, and on Facebook, you can tag pictures or post on walls). Gamasutra wants to get to the center of where exactly the interactivity lies, and in doing so, figure out what makes Warcraft a game, and Facebook a network.
One major difference is in the interface -- obviously, WoW is wrapped in a fantasy world, so that in between all of the socializing, you're also fighting the Scourge or the Burning Crusade. Facebook has games, but it doesn't have that overarching narrative. WoW also rewards group teamwork and coordination, while Facebook leaves collaboration to its own rewards. And of course the cost is another big difference: WoW is still a subscription game, while Facebook pays in other ways. But the amount of similarities between the two are pretty fascinating. And comparing the two, as Gamasutra does, really makes you think about just what interactivity means, and how two apparently very different types of interactive media aren't that far apart after all.