There is a jQuery event hooked to that link. Here is the function that is called whenever you click one of the sharing links:
window.open( jQuery(this).attr( 'href' ), 'wpcomtwitter', 'menubar=1,resizable=1,width=600,height=350' );
This is added by the JetPack plugin and will attempt to open a dialog window with the url
http://karba.la/?share=twitter&nb=1 in a window called
wpcomtwitter. The window is opening, but something is breaking inside the window such that it doesn't redirect to Twitter appropriately.
Try disabling everything but JetPack and see if that fixes it.