Having the functionality is no guarantee that anyone will use it. It's also necessary to set the norms so that people will feel the same way about updating their status on facebook as they do on twitter. This isn't to say that facebook won't do it, just that it's not as simple as adding a little code.
Umm. What functionality?
It'd be interesting to see just how much code is behind Twitter.
This issue is fundamental to the current web 2.0 startup approach of fast and cheap feature development. It becomes more and more about marketing, branding and user experience and less and less about engineering.