The impact of both Twitter and Facebook really is remarkable. While they're of course different platforms operating in different ways, each works in Social Media (posted on here) and each is a remarkable enabler of Social Media.
Facebook started first and has a wider usage (primarily for personal connections, but also with a business application (as detailed in this post on personal recommendations), but Twitter in no way should be discounted as a tool towards the Social Media idea of information dissemination.
Thing that amazes me about Twitter as an application is how it can work with such widely varied groups of people towards widely varied purposes...
If someone wants it as political vehicle... easy, get a Twitter account and follow what political figures say. If someone wants it as a tool to keep in touch with friends... great, follow them. Related to all this and in terms of Twitter as a tool for business... the audience is there, it's just a matter of being compelling enough to get people paying attention and following (or the influencers following and retweeting your message to their followers).
There's been a lot written about Twitter as a business (with this post about a Fast Company cover story), but what strikes me is my usage of Twitter as a way to learn of information held elsewhere. Basic concept is that it's all about the links. Someone you follow on Twitter links to a blog, a video, and news story... you're now taken somewhere and exposed to something that without Twitter might have gone completely under the radar.
The topic could be virtually anything (and as detailed above, it's self-selecting based on followers chosen), but the impact is remarkable... and will continue to grow in both width and depth as more people use the tool.