Noun vs. Verb
Here it goes. Twitter is an application, essentially its a thing, making it a noun. Its a thing that people go to post words about ‘what they are doing.’ That being said, when you post whatever you are doing, that is considered a ‘Tweet.”
Why?
Well once your tweet is posted, it therefore is past tense making it a verb like ate, slept, ran. You wouldn’t say I twittered that because twitter is just the application where you tweet. Basically its a fun way to say you posted whatever it was that you posted. There is a tweetie bird on the website which is the mascot for twitter, that’s also a reason why the word is accepted.