In as polite of language as possible (I guess Zuckerberg is rapidly becoming acquainted with the art of self censorship), his reply was simple: Twitter's rapid growth caused it to receive a disproportionate amount of attention. What the service does best is not something that Facebook wants to do.
This diplomatic (albeit slightly condescending) response made a distinction between Twitter and Facebook that is of particular relevance to marketers. "[Twitter doesn't] do real names, and they have themes. It's a lot more around self expression than real identity."
Indeed, on Twitter, people often are not themselves. They are an expression of who -- or what -- they want to be. They share the things that they want to define them, not necessarily the things that actually do. And often, they're sharing with strangers who are none the wiser.
On Facebook, although people certainly make decisions as to what they want to share with their networks, they do so as themselves. And the people they share with are more likely to, at least at some point, have had a real-world connection with them.
That said, are your marketing strategies for Facebook and Twitter taking into account this distinction? Or is this minute's Facebook update merely the next minute's tweet?