Only hours on the job, and industry observers are already questioning whether Dick Costolo is right for the position of Twitter’s newly appointed CEO. In fact, the world was so a-buzz with news of Twitter’s changing of the guard that the microblogging site was deluged with traffic and as a result, suffered an outage.

Costolo succeeds Twitter co-founder, Evan Williams, who abandoned his post after a two-year stint. On Monday, Williams wrote in a post on Twitter’s blog, “The challenges of growing an organization so quickly are numerous. Growing big is not success, in itself. Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world. This is no small task. I frequently reflect on the type of focus that is required from everyone at Twitter to get us there…This is why I have decided to ask our COO, Dick Costolo, to become Twitter’s CEO. Starting today, I’ll be completely focused on product strategy.”

During Williams' tenure, Twitter swelled from 20 employees to 300. Registered users increased from 3 million to 160 million, while the number of tweets skyrocketed from 1.25 million a day to 90 million. The Internet company is now valued at more than $1 billion by its investors.

To date, Costolo has played a starring role in turning Twitter into a promising advertising vehicle – efforts that include spearheading the launch of Promoted Accounts. The service suggests accounts that people don’t currently follow and may find interesting. For example, when an advertiser promotes an account, Twitter’s algorithm looks at that account’s followers and determines other accounts that those users tend to follow. If a user follows some of those accounts, but not the advertiser’s account, then Twitter may recommend the advertiser’s Promoted Account to that user.

Costolo, the former co-founder of Feedburner, which was acquired by Google in 2007, was brought onboard Twitter as COO a year ago.

In the meantime, Skype is also replacing its CEO, Josh Silverman, with Tony Bates, a former Cisco senior vice president. Bates will start at the end of October; until then, Skype chief financial and administrative officer Adrian Dillon will serve as interim CEO.