BuddyPress.org

I have a little gift for you.
Add this code in your functions.php

function my_test($args)
{
	if( is_user_logged_in() )
	{
		global $activities_template;
		$logged_user = bp_core_get_user_displayname( bp_loggedin_user_id() );
		if (strpos($args, $logged_user) !== false)
		{
			$old_text = '>' . $logged_user . '</a>';
			$new_text = '>' . __('You', 'buddypress') . '</a>';
			$args = str_replace($old_text, $new_text, $args);
		}
	}
	return $args;
}
add_filter('bp_get_activity_action', 'my_test');