You may add the new block to the home page with the following MySQL command:

INSERT INTO `sys_pages_blocks` (`object`, `cell_id`, `module`, `title_system`, `title`, `designbox_id`, `visible_for_levels`, `type`, `content`, `deletable`, `copyable`, `active`, `order`) VALUES

('sys_home', 1, 'bx_persons', '' , '_bx_persons_page_block_title_latest_profiles', 11, 2147483647, 'service', 'a:2:{s:6:"module";s:10:"bx_persons";s:6:"method";s:22:"get_online_friends_num";}', 1, 0, 0, IFNULL(@iBlockOrder, 0) + 1);

The bold words give the name of the new service method from the BxPersonsModule.php file. It should have the name like:

public function serviceGetOnlineFriendsNum () { code of your method }