Seems Twitter may be the "next big thing" in social media. Hitwise reports it has surpassed Digg for usage - but realistically the two are different types of social media, though Twitter does have the recommendation aspects of Digg. Comparisons to Facebook may be more realistic.
Twitter has come to replace - or at least heavily supplement - the interactions between Facebook users via leaving messages on the Wall of each user. The big difference about Twitter is everyone who is following a person gets to see the messages sent and join in the conversation immediately.
It combines the community nature of Facebook with followers and there have been requests to start groups similar to those at Facebook and other features. But at the moment the rising number of users has been causing problems with overload.
Twitter doubled its resources for yesterday's inauguration, knowing people would log on to share comments, yet still had some problems.
If Twitter is going to continue to grow it will have to overcome more than its capacity issues. Methods of monetization are a major hurdle, but meeting the needs of greater features is the biggest one. Rivals have popped up and have yet to impact. If one can address the other features issue they may have better luck.