As most informed citizens of the world now now, Iran is having a lot of turmoil over its recent elections. The most interesting part about it (ok perhaps second most interesting after the chance of a potential revolution in Iran) is the power of the internet and how news and information has traveled. The story was largely ignored by major news networks (looking at you CNN) but it gained life on Twitter (@ProxyHost now on twitter). They re-scheduled maintenance around the event and news from Iran has been all but cut off with the exception of Twitter.
Why is Twitter the exception?
*Open network - Anyone can join
*Open API - Tons of applications, services and proxies can access and post information
I think those are the two main reasons. The first gave rise to a service and the second cemented it's power for the users. With a completely open API anyone around the world can access it, build apps and allow other services to interact on their behalf. So let's say government Y blocks access to twitter.com, they still have to block every proxy (web service that allows you to interact with the service/post information to it/etc) to effectively shut the service down. Now that Twitter has grown and developers have taken the challenge of building applications for it on, it simply can't be stopped. News, leaks and videos pop up onto the service constantly no matter how hard a government tries to stop it.