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.
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.
What's better: a cold, formal tone that is very professional, or one that is casual and makes you feel like an actual person is behind the monitor?
It's all a matter of opinion, and definitely depends on what you are selling. Banks? Definitely shouldn't be doing that.