That only applies when the relationship implies bi-directionality. On Twitter, me following you doesn't necessarily mean that you follow me. Thus both directions are persisted in the model.

If this were on Facebook, however, you would be right. (:User)-[:FRIENDS_WITH]-(:User) would only need to be persisted once and with an arbitrary direction.

I agree if you're modeling a symmetric friendship network like Facebook. However, Twitter following can be asymmetric. I could store the directionality on a relationship property, but that gets even uglier.