At some point yesterday, Twitter surpassed the 10 billion Tweets mark, which, you’re no doubt wondering, means what exactly? Well, it’s just another measure to tell us what we already know: the micro-blogging platform is now mainstream.

Consider these quick observations: usage (by volume of Tweets) doubled in the past four months and has grown 10-fold in about 15 months (from 1 billion in November, 2008).

At the same time, corporate Twitter usage is growing, though much more slowly. According to a new study, 27 of the Fortune 100 companies now have an active Twitter feed.

From our discussion with companies, many more are soon to follow, or are at least considering a corporate Twitter policy. Which ones? In the UK, think an entire sector of the high street landscape that is not yet on Twitter: yep, the banks. When they do make the plunge, we advise they look at our Ten Commandments for Corporate Tweeting.