I reply to myself. I don’t know if it’s the most ortodox way (I’m sure it’s not) but it works 
In /_inc/bp-follow-hooks.php, function bp_follow_add_activity_scope_filter:
`$following_ids = bp_get_following_ids( array( ‘user_id’ => $user_id ) );
$following_ids = $following_ids . “,” . $user_id;
`
BTW, I still don’t know why BP is using strings instead of array of integers for handling user IDs.
Hope it helps someone too.
This might help you out: http://buddydev.com/buddypress/facebook-style-activity-commenting-with-buddypress/
How did you manage to get the Activity Stream to default to Followers @abysshorror ?
Hey, thanks for answering ! it’s really hard to get someone to answer around here, specially this days 
To get the activity stream default to their followers I used this:
http://wpmu.org/daily-tip-how-to-set-buddypress-profiles-as-the-hompepage-for-logged-in-users/
And changed the code a bit:
`function bp_profile_homepage()
{
global $bp;
if(is_user_logged_in() && bp_is_front_page())
{
wp_redirect( $bp->loggedin_user->domain . ‘activity/following’ );
}
}
`
If you are still having problems, ping me !