Twitter. Twitter. Twitter.
Not X.
Not X.
See also: Google/Alphabet, Blackwater/MurderInc/Whatevertheyrecallednow
Though for me personally, if someone changes their name, I usually try to call them by that name. But usually not the easiest, I admit.
Socially they are not.
I feel no social obligation to cooperate with a private for profit corporation's marketing strategy. If I ever have legal documents relating to Twitter I will use whatever their legal name is. Until then, they're Twitter.