Iran shut down access to both Facebook and Twitter during the 2009 post-election troubles and both have remained inaccessible, even though President Rouhani has a Twitter account.
Until today, when Iranians discovered they could access both sites. There has been no announcement so it's unclear whether this is just a glitch of some sort or a quiet bit of liberalization. It isn't clear if it's working throughout the whole country or even on all ISPs. Or, of course, if it will still be there tomorrow.