At the risk of sounding like an old fart, the idea that some people "tweet for a living" gives me one of my "maybe al-Qa'eda has a point" moments.
i think it's more the case of marketing/PR people who, as part of their overall job, have to send tweets on behalf of their clients. it's not like the CEOs of stubhub or kitchenaid are writing/sending those corporate tweets.