It is easy for a government to block email or a website. And they know why they require internet and mobile phone providers to integrate the necessary equipment into their networks.
But it is almost impossible to block communication thru Twitter, combined with YouTube and Facebook, as the Iranian government learnt last summer.
Why is this?

Twitter, [...]

There are two types of traffic:

Traffic which comes from internet users who find you thru search engines, or by following a link, paid or unpaid, to your site.
Traffic coming from mailing lists or socal networks, which you can induce by sending an email to your list, or by making a post to facebook, twitter [...]