Not to my knowledge, however it's entirely possible. It wouldn't be a fully-fledged facebook on the DS, but more like the mobile version where you can check your wall/update your "What's on your mind"/upload photos and such. Heck I'm sure if at some point along the line homebrewers manage to get ahold of the DSi camera for their own purposes you could easily take and upload photos on the fly with a DSi. Facebook does have an external API developers are allowed to use, however I'm not sure of the practicality of getting the DS to interact with it. More than likely there'd be some server on your PC that would interact with the API for the DS.
Certainly less advanced to do a mobile re-creation of facebook for the DS than some of the other awesome homebrew I see out there, and the DS is a hell of a lot more powerful than my phone and my phone can still interact with facebook.