Back in September, Twitter announced it had 145 million users. Staggering, no? Well two months later, it now has 175 million users. Some are predicting it could break 200 million by the end of the year.

Quick math: that's 30 million new users in less than 60 days. More involved math: that's 370,000 new users signing up each day!

There's really fast growth and then there's meteoric. Twitter's growth is truly meteoric.

Something else amazing has happened to Twitter during this same time period. About a month ago, Twitter co-founder, Evan Williams, voluntarily stepped down as CEO handing over the wheel to his then COO, Dick Costolo.

Williams admitted he wasn't the guy to run Twitter.

In an article that ran in the NY Times this past weekend, Williams and his colleagues paint a pretty clear picture of his shortcomings as a CEO:

1. He isn't detail oriented.

2. He prefers to work alone.

3. Even Williams himself admits in hindsight that he was slow to hire up when the company was mushrooming in size and recruit the level of talent needed to grow with the company.

In short, he's not much of a manager. All this and to boot, he's not the most inspiring public speaker by most accounts.

Having a great idea and starting a business is one thing. Running a large company is another. Two years ago, when Williams became CEO of Twitter it was still a SMB with 20 employees. I think it is safe to say it is no longer a SMB.

All that being said, Williams would get my vote as one of the most effective CEO's of 2010. His greatest decision came from the wisdom of knowing when to step aside.

My question to all of you start-up CEO's, do you know when to say when? Will you recognize when your company needs a different set of leadership skills than the ones you bring to the party now?

Speaking of Twitter, feel free to follow me @oricchio

(one of 175 million and counting)