Twitter (and related status-sharing services) has grown tremendously since it's beginnings in 2006. In late 2009, the service gained credibility and mainstream adoption after many celebrities began using the service to send updates to their fans.
Twitter's strongest draw is its ease of use, enabling users to quickly jump in and start 'tweeting.' There is little to no information required of a Twitter user, and there are literally hundreds of ways to send updates to your Twitter account.
It is also very easy to follow others and be followed. Many organizations and individuals have hundreds or thousands of Twitter followers, and they typically don't have to try too hard to keep the followers once they have them.
However, effective Twitter use, unlike Facebook, requires more vigilance and more personal investment. Most organizations that use Twitter effectively allow individuals in their ranks to Tweet on behalf of the organization. And those that have organizational accounts typically have individuals who frequently check whether their account has received any "@" replies, direct messages, or if any updates have been "retweeted" (e.g. 'liked') by other Twitter users.
To become popular on Twitter, one must invest the time to reply to others in a personal manner, and must also make sure that new tweets are posted on a timely basis—three to five a day is not unusual for most organizations.