I have the feeling Facebook's leadership is dramatically pragmatic: if it works, let's use it. They borrow ideas from lots of people, "like" came from friendfeed I believe? And Twitter's leadership seems more, how shall I say it, more interested in doing it "right" or taking the high road in terms of features. They have stayed extremely minimal for such a popular service, in terms of features.

They could add "like", which would surely add value, but then again they could also add a dozen other features that add value. But in doing that, they'd dilute their simple value proposition: real time updates.

Whether laser-focusing on 1 feature/value is smarter than adding other features and having a more complete offering is smart, only time will tell, I'm really not sure at this point.