bbPress.org

It’s because you are destroying $user_id

This is wrong:

$user_id = bb_get_user( $user_id );

$bb_twitter = $user_id->social_twitter;

You should do:

$user = bb_get_user( $user_id );

$bb_twitter = $user->social_twitter;

$user_id holds the id number, not the entire user object.