Too casual puts me off (HP has a person who makes me never want to buy from them again), but if I don't get a sense of "real person" from you, I think you are lying to me.
Social media is about people interacting, even when those people work for huge companies. Talking with the same stilted, jargony crap that shows up on the web site is worse than not communicating at all. And that's what corporate standards tend to breed.
I think one of the companies that uses social media well and without that 'oh too cool' casual style is mediatemple. I find their tweets and video posts to be professional and personable. Even when other twitterers are screaming at them when one of their shared servers goes down.
There is an immediate drop in creditability when a business is too friendly or too casual. I don't think you need to be cold and formal to portray a sense of responsibility and professionalism. It's about striking a balance between the extremes and reflecting the type of business you are.
Presumably the point of the betfair twitter stream mentioned http://twitter.com/betfair is to get an audience, any audience. It is pretty bizarre. Cant imagine most companies agreeing to it. More interesting than the cutesy style that gets annoying very fast.
I blame Pret A Manger. They have been doing this for years, I think the goal has been to make you think it is not a store, but a collection of foodstuffs curated a manic pixie dream girl, and oh yeah, you can totes buy the bananna bread! It is sooooooo good!
Social media is about people interacting, even when those people work for huge companies. Talking with the same stilted, jargony crap that shows up on the web site is worse than not communicating at all. And that's what corporate standards tend to breed.