Facebook just announced a public API to the Facebook Stream (aka Facebook updates). This adds to the already sizable API's available via Facebook and takes them into direct competition with Twitter, who's success can be at least partly attributed to their open API and the plethora of Twitter driven application that are now available.
I haven't yet had a chance to review the new API, but I am excited about it. Among other things Facebook allows for conversation threading, a big piece of what I think Twitter lacks. The API has already been available to a select group of Beta developers, one being the Adobe AIR based Seesmic Desktop. Facebook has also released their own AIR application as well.
Twitter lookout! Here comes Facebook...