Twitter business plan convincing some people

Today's announcement that Twitter has secured another $55m in venture capital suggests that whatever the revenue generation plan is, there are people willing to invest in it. Within the announcement was the detail that only around a third of Twitter traffic is on their website: the majority is through the API, which suggests where revenue generation is going to be focused. That feels important, in the same way that Twitter feels important. So as there isn't an easy elevator pitch for Twitter I'm finding myself trying to explain it more and more to colleagues, friends, and random passers-by that I have to apologise to when I bump into them while on Twitterberry. Just saying 'Facebook status on IM' doesn't really work any more; it feels like a copout, as even the people who don't understand Twitter know it is more important than that. Likewise I can't guess exactly what the business plan is, but it is clearly all about the service not the site.