I agree its great to allow people to connect, I already managed this with
caribbean-diving.com
This just creates a new user in joomla however and I doubt it will work with k2 though k2 has a plugin i see that is supposed to work with synchronising users ?
Still the other thing once the user accepts to share with facebook is actually doing the facebook post - this is just one line of code believe it or not.
The trouble with facebook is they change their API so often it will cause a lot of maintenance work to be needed.