Get the most recent post of a user.
Walks through each of a user's blogs to find the post with the most recent post_date_gmt.
Works on Multisite only.
<?php get_most_recent_post_of_user( $user_id ); ?>
Contains the blog_id, post_id, post_date_gmt, and post_gmt_ts
get_most_recent_post_of_user() is located in wp-includes/ms-functions.php