SugarCRM Notifications like facebook

custom_modules_Activities_ActivitiesLogicHookCustom.php

<?php
class ActivitiesLogicHookCustom{
    public function notifyActivity($bean, $event, $arguments){
        if($bean->module_name == 'Activities'){
            global $current_user;
            $data = json_decode($bean->data);
            $message = preg_replace('/(\@)\[\w+\:[a-f0-9-]{36}\:([\w ]+)\]/u', '$1$2', $data->value);
            foreach($data->tags as $tag){
                if($tag->module == 'Users'){
                    $notification = BeanFactory::newBean('Notifications');
                    $notification->name = $current_user->user_name . ' mentioned you in an activity';
                    $notification->description = $message;
                    $notification->assigned_user_id = $tag->id;
                    $notification->save();
                }
            }
        }
    }
}

custom_modules_logic_hooks.php

<?php
$hook_version = 1;
$hook_array = Array();
$hook_array['after_save'] = Array();
$hook_array['after_save'][] = Array(1,'Notify activity','custom/modules/Activities/ActivitiesLogicHookCustom.php','ActivitiesLogicHookCustom','notifyActivity',);

You should move your custom_modules_logic_hooks.php to
custom/Extension/modules/Activities/Ext/LogicHooks/notify.php