Most Helpful Guy
The only time it's okay is if it's a scenario like yours. If they're abusive, especially physically, then yes, a text is a good idea. Don't have to deal with the after drama of a barrage of negative and hurtful words and possible physical violence. You're in the right. As for the Facebook situation, you and your friends need to unfriend and block him because that behavior is unexcusable and deplorable.
Most Helpful Girl
With the writing on the wall and all here, dear, it is only right now To... Put this Problem pisser and Problem child on your own Pay no mind list and don't bother with him again, my friend.
Normally, when in a relationship, I would say don't Text, face to face even, to say "Goodbye, be on your way."
However, this loser does Not 'Deserved more' and with you dropping him like a Hot potato will make him look even more stupid now because after you BLOCK HIM on FB as well, he can Go... To hell.
Good luck. xx