gistfile1.php

<?php

global $wpdb;

$results = get_transient( 'hub_listed_blogs' );

if ( false === $results ) {
    $results = $wpdb->get_results( 'SELECT blog_id, domain FROM wp_blogs', ARRAY_A );

    // cache for a week
    set_transient( 'hub_listed_blogs', 2 * WEEK_IN_SECONDS );
}

if ( 0 === count( $results ) ) {
    return;
}

$latest_posts = get_transient( 'hub_latest_posts' );

if ( false === $latest_posts ) {

    foreach ( $results as $blog ) {

        switch_to_blog( $blog['blog_id'] );
        $latest_posts[] = hmn_get_latest_post( $blog['blog_id'] );
        set_transient( 'hub_latest_posts', 1 * HOUR_IN_SECONDS );

    }
}

// Return the latest post for a blog
function hmn_get_latest_post() {

    $args = array(
        'posts_per_page' => 1,
        'ignore_sticky_posts' => true,
    );

    $latest_posts = get_posts( $args );

    wp_reset_postdata();

    if ( 0 < count( $latest_posts ) ) {
        return $latest_posts[0];
    } else {
        return false;
    }

}