That’s a core function and it was moved into a new file, wp-includes/post-formats.php, for 3.6. Is it possible that you didn’t have that file? I don’t think this could be an issue with Pinboard.
Just to add a data point we have multiple multisite instances with 2.5.2 installed, in both WP 3.5.1 and WP 3.6 beta 2, and we haven’t seen this problem. Based on problems I’ve seen with other WP plugins I suspect there’s a plugin throwing javascript errors which screws up the AJAX functionality of this plugin. It wouldn’t be the first time nor the last.
