An early-adopter friend of mine, way back in '07 or '08, announced by Facebook that he was Twittering. At the time I hadn't heard of Twitter, and worried that he was publicly confessing he had developed an amphetamine habit. But we don't use Twitter as a verb. We tweet and re-tweet, right?
Actually, we do use Twitter as a verb - when we're speaking about
habitual use. "I'm Twittering" means "These days, I'm using Twitter" as opposed to "I'm sending a message on Twitter at the moment" - that latter sentence would be abbreviated by saying "I'm tweeting."
Many languages preserve a morphosyntactically encoded distinction between present and habitual - in fact, that's even one distinction between standard American English and the oft-villified ebonics (ebonics has it, standard Am English doesn't.) Here it's marked by requiring separate verbs. Minor observation, but interesting nonetheless.
Modality is a bitch (updated)
10 hours ago