Gets a user’s most recent post.

Walks through each of a user’s blogs to find the post with
the most recent post_date_gmt.

function get_most_recent_post_of_user( $user_id ) {
	global $wpdb;

	$user_blogs       = get_blogs_of_user( (int) $user_id );
	$most_recent_post = array();

	// Walk through each blog and get the most recent post
	// published by $user_id.
	foreach ( (array) $user_blogs as $blog ) {
		$prefix      = $wpdb->get_blog_prefix( $blog->userblog_id );
		$recent_post = $wpdb->get_row( $wpdb->prepare( "SELECT ID, post_date_gmt FROM {$prefix}posts WHERE post_author = %d AND post_type = 'post' AND post_status = 'publish' ORDER BY post_date_gmt DESC LIMIT 1", $user_id ), ARRAY_A );

		// Make sure we found a post.
		if ( isset( $recent_post['ID'] ) ) {
			$post_gmt_ts = strtotime( $recent_post['post_date_gmt'] );

			/*
			 * If this is the first post checked
			 * or if this post is newer than the current recent post,
			 * make it the new most recent post.
			 */
			if ( ! isset( $most_recent_post['post_gmt_ts'] ) || ( $post_gmt_ts > $most_recent_post['post_gmt_ts'] ) ) {
				$most_recent_post = array(
					'blog_id'       => $blog->userblog_id,
					'post_id'       => $recent_post['ID'],
					'post_date_gmt' => $recent_post['post_date_gmt'],
					'post_gmt_ts'   => $post_gmt_ts,
				);
			}
		}
	}

	return $most_recent_post;
}