ThreeWP Activity Monitor Version 2.3
Locate this code in file ThreeWP_Activity_Monitor.php near line 1030:
$post_link = $bloginfo_url . '?p=' . $post_data->ID;
Replace with this:
$post_link = get_permalink($post_data->ID);
Note: Already saved activities will still have the old format, only new added activities after this patch will get the correct permalink.
Also, the ?p=id link-format does not support pages or custom post types. Version 2.3 of the plugin actually does not support these, but if you use the patch for pages and custom post types, you want to use this permalink-fix as well.