Twitter successfully incorporated components of other growing social media tools, like social networking from Facebook and link sharing from Digg. Maybe most importantly was Twitter's ability to easily operate the social tool though mobile devices. The mobile nature of Twitter caught on quickly, with multiple mobile applications being created for users on different networks and with different types of phones. Though the tool itself was exciting and new, Twitter didn't take off until celebrities moved onto the platform.
It was estimated that over half a million new user accounts were created in the two days following Oprah's announcement that she was joining the micro-blogging platform. This was an increase from the 5,000-10,000 new account created every day prior to that.
There is no question that Twitter created a fantastic tool that appeals to the current culture of communications, but I sincerely doubt it would be as popular as it is today in such a short period had it not been for celebrity involvement.